according, I believe, to Roman Catholic usage of the word. Evidently, this means in a special ecclesial way, since the Holy Spirit lives in us by virtue of our Baptism, inspires and co-ordinates the strands of our intelligence and understanding, and prompts us to pray. So, in that sense, a sacrament is much, much more than a sign of God's love.
Of course, like Baptism, itself, the Holy Spirit's effect's can be effectively rejected by serious sin, as you imply in your last sentence. For a marriage to be valid and real, sacramentally, there has to be a "one to one" commitment between the partners. Immaturity on the part of just one partner, resulting in the absence of a "one to one" commitment renders the sacrament null, from the sacramental standpoint (although not secular law, which remains valid). It was meaningless and constitutes grounds for a formal Church annulment.
An odd thing about the marriage of a Christian couple, as a sacrament, is as follows:
"The Ministers of the Sacrament:
How can a marriage between two non-Catholic but baptized Christians be a sacrament, if a Catholic priest does not perform the marriage? Most people, including most Roman Catholics, do not realize that the ministers of the sacrament are the spouses themselves. While the Church strongly encourages Catholics to marry in the presence of a priest (and to have a wedding Mass, if both prospective spouses are Catholic), strictly speaking, a priest is not needed.
The Mark and Effect of the Sacrament:
The spouses are the ministers of the sacrament of marriage because the mark—the external sign—of the sacrament is not the wedding Mass or anything the priest might do but the marriage contract itself. This does not mean the wedding license that the couple receives from the state, but the vows that each spouse makes to the other. As long as each spouse intends to contract a true marriage, the sacrament is performed.
The effect of the sacrament is an increase in sanctifying grace for the spouses, a participation in the divine life of God Himself."
This is an excerpt from the following website:
http://catholicism.about.com/od/beliefsteachings/p/Sac_Marriage.htmBaptism, is also an odd sacrament in that, in an emergency, it may also be administered by a lay-person.