Your suggestion that marriage is simply about sex only supports the idea of same sex marriage-equality. It's a shallow view, IMO, and it plays into the hands of those that want to devalue marriage.
We're not talking about just sex. We're talking about a complicated relationship between people and their relationship to the rest of society.
Proper family formation is essential for the healthy continuation of society and civilization. A special social recognition and support of those people that wish to begin families is important.
Agreed, and I think, a major reason for the seeming generational split in who supports homosexual marriage. Many young people, including those who consider themselves Christian, don't see what's wrong with homosexual marriage, because they see marriage as being about feelings, love and sex. What's interesting is that many of the young people who support homosexual marriage have no interest in marrying themselves. They have a very jaded view of the institution, caused by the frequency of divorce over the past 30 years, so they think "why not" when homosexuals demand their 'right' to be married.
Much of this is because of the way the twenty and thirty somethings are viewing children. Many of them see children as impediments to their fun, or their careers, etc., so they don't see marriage and children as being connected, anymore. Easy access to contraception, and the general liberal attitude toward sex that has been created by the media have contributed to this. Easy divorce, and what they see of how heterosexuals have damaged the whole notion of marriage has only contributed to their ideas.