THe real point is that Marriage is a Sacrament, and was instituted by God himself, as with the other Sacraments. As such, it's not open to redefinition. No analogy applies here, whether it's slavery or any other moral example.
There is a guy at my parish who is homosexual. It opened my mind to actually know someone who struggles with their sexuality. I can understand how difficult it is. I was once very prejudiced but have come to empathize a great deal. It's hard afterwards to look at them and say "those people". I believe he has the best and most balanced way of approaching it that I've seen.
It should be said that sexual orientation (heterosexuality/homosexuality) is different from sexual activity. The former describes who someone is innately attracted to. I don't know if it's a mental problem or not, but there is by and large no control over it, any more than a person chooses what flowers they think smell good. It is what it is. The latter refers to what they then do with their attractions, and that's what the Church speaks to. It's important to make that distinction.
My friend would like to have a relationship with another man, but knows the Church doesn't bless that, because marriage is a Sacrament. He must die to his own desires and learn to find that fulfillment in God. That's what we're all called to do.
A Christian is first and foremost called to holiness, not happiness. Someone may desire to get drunk, or use drugs, or eat too much, or pleasure themselves, or have homosexual sex, or any number of things, but the Church will not bless it. No Christian should weasel a way to fulfill his own desires. Life is either more and more about oneself, or more and more about God and others.
This goes to the Church's reasoning behind the issue, from what I understand - that a homosexual relationship cannot produce children (also the reason for prohibiting contraception). Sex can either be selfish or selfless. If it's just for pleasure, it's selfish and therefore wrong. If it's for continuing Creation, it's selfless and therefore good.
THere's much more to it than some ancient heterosexual men saying "that's disgusting." Sometimes advocates of gay marriage in the Church present it that way, and it's not true. It's about the relationships with God, ourselves, and Creation.