What is essential to marriage? It is essential to marriage that the couple, by being the kind of couple it is, is ordered towards having children. That is the basic essence of marriage.
A couple that consists of a man and a woman, by being the kind of couple that it is, is ordered towards having children. This is true even is this particular couple, consisting of this man and this woman, is not capable of having children due to age or infertility. It is even true if this man and this woman use contraception to prevent themselves from having children. The universal, i.e. being this type of couple, is essential. The particular, i.e. being this particular couple, is accidental.
A couple that consists of two men or two women is not ordered towards having children. This is true by virtue of being the kind of couples they are. Neither two men nor two women are capable of making a baby. This is not due to being this particular couple, but to being this type of couple. It is not accidental, it is essential. As such, neither two men nor two women are capable of marriage.
I hope that clears it up for everyone who tries to argue for same sex "marriage" by arguing that we do not make heterosexual couples prove they will not use contraception before the government considers them married. Such arguments are fallacious. They are examples of the fallacy of the accident.