First remember, the LEGAL rationale being used to require States to treat Homosexual couples like heterosexual married couples is equal protection of the laws. i.e. The state can NOT treat two people in the same circumstances under the law. The issue is when is the state treating people differently? First the Federal Court have always held the Equal Protection Clause does NOT cover economic differences (leading to the famous observation that it is equal protection of the laws to forbid both the poor and rich to sleep under bridges, knowing full while only the poor are force to sleep under bridges do to economics while the rich can afford someplace better).
As I said, the Equal Protection Clause does NOT cover economics. Thus what does it cover? The intention was to force the South after the Civil War to treat the recently freed slaves the same as whites. Thus the STATES could not write laws that were aimed at blacks but not whites. Now the south used Separate but Equal for over 50 years to get around this requirements but finally in 1954 the US Supreme Court ruled it was a violation of the Equal Protection Clause of the Constitution to Separate Blacks from whites by law. The courts then took this argument that the States could NOT forbid blacks and whites from marrying. Then took it to prohibit ANY disparateness in treating two people in similar circumstances (Except if the difference is based on Income).
Now Europe is NOT using the Court system and any clause in their Constitutions like the Equal Protection Clause to permit gay-marriage, Europe is granting such marriages by STATURES i.e. the various Parliaments are passing LAWS to permit such marriages. That is NOT happening in the US, the thrust for gay-marriage is through the courts and the Equal Protection Clause NOT through the States Legislatures. The reason for this seems to be more to do with the much more frequent elections in the US compared to Europe, the Parliamentary system that REQUIRES a majority to maintain any Government and the separation of powers in the US (Europe believes in Parliament supremacy).
Any back to the Equal Protection Clause. This is the main thrust for Gay-marriage in the US, a legal Argument that the States can NOT discriminate against tow similar situated persons in regard to whom should they marry. In the 1960s this argument was used to strike down the miscegenation laws that forbade whites and blacks to marry on the grounds you can NOT treat blacks and whites differently, thus is a black wants to marry a white the state can not forbid such a marriage under the Equal Protection Clause.
Now, the equal Protection Clause has always permitted different treatment between the Sexes when the state has some good rationale for the different treatment (For example the State can forbid men and women from using the same restrooms). Now, their has been some contention in this area of the law, for example can the state forbid women from going topless while permitting men to go topless? On the face the answer should be no but the Courts have avoided the subject, aided by the fact most women do NOT want to go topless in public, other than on beaches (and some old laws rarely enforced that forbid men to go topless). The Courts biggest problem is the fact women can breast feed and men can not. This come sup in Custody cases where a women is breast feeding her baby. How do you give equal time to the father when Doctors states Breast feeding is good for the baby, Not-breasting feeding will dry a women breast's up and the father only way to "breast-feed" is considered second best by most Doctors (i.e. the "Breast pump" and other artificial Breast feeding devices)?
A third set of problem is the difference between strength in men and women. Is it fair to permit men to justify not hiring a women for she can not left as much as a man when such lifting is rarely done on a job? What about the fact women's hands are substantially smaller than a man and thus easier to to get into small openings? (Sewing is the most obvious case but they are other cases).
Thus the Courts when it comes to Equal Protection of the Law have had to handle the PHYSICAL difference between men and women. Between men or between women, the Courts have adopted a very strict rule, no discrimination unless the state can show extremely good reason for the difference treatment (and the only case the Supreme Court upheld on this grounds was the Japanese internment of WWII). On the other hand discrimination between women and men have always been treated with an easier rule for the state to meet. i.e. More than any rationale reason but less than the strict forbiddence, if the state can show a good justification for the difference in treatment.
Thus the Court will have a problem using the equal Protection Clause to require the states to permit gay-marriage. Right now most Courts rely on the fact that at the time of the Adoption of the Equal Protection Clause not as single congressmen thought or stated that the Equal Protection Clause would cover sex discrimination. In fact people at the time OPPOSED the Amendment for it is the first time in the Constitution that the term MALE is used (and that only is regard to Voting) and women suffrage Groups wanted to use the 14th Amendment to give women the right to Vote. Thus it is clear the authors of the Amendment accepted different treatment between men and women.
So far the States have avoided using the FEDERAL Equal Protection Clause for it is clear the Federal Courts will rule it is NOT a violation of the Federal Equal Protection Clause for a state to treat sex differently than race. The Federal Courts have ALWAYS treated sex differently than race. Thus it is NOT a violation of the Federal Equal Protection Clause to forbid all marriages between people of the same sex.
On the other hand, once a state permits same-sex marriages the Federal Equal Protection clause does kick in if you treat people of the same sex differently. For example this case, if these two women were Lesbians living in the US in a state that permitted same-sex marriage, how can they be permitted to marry when two sisters who are lesbians can not? If two Lesbians sisters can marry, how can you treat them differently than two sisters who do not have sex with each other? The traditional rationale against blood relatives being married (i.e. inbreeding) does not apply for you can not have children if the sex partners are of the same sex. To Keep in-breeding to a minimum is the reason people can not marry siblings, but if the siblings are of the same sex in-breeding is not a concern. Thus how can you discriminate against two people of the same sex to marry if the only reason is one set is blood relatives and the other is not? Sorry you have two people of the same sex being treating DIFFERENTLY for no rational reason. The federal Equal Protection Clause would forbid such discrimination.
Now both Vermont and Massachusetts have used States Constitutional Equal Protection Clauses to rule that the State could NOT prohibit Gay-Marriage if the State permitted Straight Marriages. Now this case will show people a way to show discrimination between two people of the same sex under the FEDERAL CONSTITUTION. Will the business community accept this? Remember we are talking about expanding the right of "Marriage" to any two people living together for any reason.
Now the Supreme Court likes "Bright lines" when it comes to the Equal Protection Clause. Traditionally the Bright lines when it comes to households were Blood relatives and people of the Household. People are often shocked when I tell people a Spouse is NOT an Heir of a Spouse, for the spouse is NOT a blood relative. Under the Common Law the rights of a Spouse were set in Law BUT ONLY IF A CHILD WAS BORN TO THE RELATIONSHIP (even if that child subsequent died). Thus a sort of blood relationship existed even between spouses in the form of their Children (Something homosexual still can not do, even in artificial insemination either the ovum or sperm of someone outside the relationship has to be used).
The other Bright line was "Of the House". This covered people who lived in the house of the Lord. What these people did was held to be the action of the Lord, thus if a servant of the house caused someone else injury, the victim could not only sue the servant but his master also. This survives in Employee-Employer liability. If someone under your employ cause someone harm, the employer is liable. I bring this up for if you start to get to far away from straight marriage I can see the Supreme Court ruling that the next bright line is people living in the same house (I can even see Scalia and Thomas voting this way knowing in many way they would be killing Gay-marriage). Once the Court rules that the next bright line is "On the house" even people living in a Commune setting would be treated as "Spouses" for employee benefits. The business community, while willing to accept the very small price of providing such benefits to homosexuals, will refuse to expand such benefits and will lobby the states to ban Gay-marriage so the Expansion to the the house will also be banned.
This expansion and then contraction will take a while, giving the GOP plenty of years to Gay-bash, but sooner or later the law will return to what it once was for the Business community is willing to pay for spouses and children of their workers medical care, but not for everyone who happens to move in with one of their employees.
14th Amendment:
http://www.law.cornell.edu/constitution/constitution.amendmentxiv.html