Homosexuality is NOT mentioned in the ten Commandments. Now under Catholic Doctrine (A term I would avoid if I were you with a Fundamentalist, Catholics are not Christian in the eyes of many Fundies) the only truly "Mortal" Sins are violation of the Ten Commandments, other "Sins" are NOT "Mortal" or "Severe" (In the old Baltimore Catechism the term "Mortal Sin" was used but in the present Catechism the term "Severe Sin" is used, both are restricted to violation of the ten Commandments).
In Catholic Doctrine all other sins are minor sins. A better term than "sin" would be "error" for to many people hear "sin" and get defensive. You have to explain to them that "to sin" is "to error" and everyone errors (and thus everyone sins). This is basic Catholic Doctrine and most Christians follow this difference between Severe and Non-severe sin to some extent.
One way to bring this up is to ask the person what is the difference between a violation of the Ten Commandments and a sin (such as Homosexuality) that is NOT a violation of the Ten Commandments? Most fundies either do not accept the difference (than thus why did God provide the ten Commandments?) or they claim some other part of the Bible describing a Sin as making that Sin the same as violating the Ten Commandments (at which point you go back to why is the Ten Commandments the Ten Commandments if God made other Commandments?).
The Catholic Church has resolved the above "problem" by simply saying the only sins you can be damned to hell for are Violations of the Ten Commandments, all other sins must be weighed with your good deeds (a Topic to stay away from with Fundies, they do not believe Good Deeds gets you into Heaven, just Faith even through Jesus himself says faith with out deeds is not enough).
With Fundies stay with the Ten Commandments, ask them why did not God put Homosexuality in the Ten Commandments if God wanted Homosexuality punished like the rest of the Ten Commandments? Stay with the Ten Commandments and what is NOT in them. It blows the Fundies mind that the Ten Commandments does NOT address any sex crime except Adultery (Unless the act violates some other sin in the Ten Commandments for Example when St Paul condemned people who committed Homosexual acts he was referring to people who went to a pagan temple and paid for such acts with a slave of that temple, the crime was not the Act but that the person one had sex with was being forced to have the sex with you. You thus violated his or her right to her own body, a right which under Roman and Greek laws did not exist for slaves but none the less was viewed by St Paul as violating the Commandment to honor God, to honor your Parents, to honor one's neighbor etc. It further violated Jesus commandment Not to do on to other that one does not want done to you. Forced sex violates all of these and that is what St Paul was condemning NOT homosexuality itself).
Thus point out and use the Ten Commandments and Jesus statement to love your fellow God and your Fellow man as you love yourself.
More on the Ten Commandments:
http://www.newadvent.org/cathen/04153a.htmhttp://en.wikipedia.org/wiki/Ten_Commandmentshttp://www.religioustolerance.org/chr_10co.htm