The real modern terms of marriage aren't those of the church or the Pope or any religious leader.
The real modern terms is how most people from the newest generations look at it.
Which is:
Man + Woman = Normal Love
Man + Man = Woman + Woman = GROSS YUCK!
We can disagree with the opinion, but we can't disagree that the opinion is there.
Don't make me get all physicalist on you...
Anyway, just because some people don't like the idea doesn't mean gays should be denied rights. I think the main reason gay marriage is allowed now from what I've heard is the legal perspective.
I'm not sure how inheritance works in the US and other countries but in Britain if you die without a will all of your possesions go to your spouse fist, then your children if there is no spouse, and then to a distant relative. If there's no one the state takes it in the same way as tax. The problems that arose before gay marriage was allowed involved the following scenario:
A gay couple live together but cannot marry. One part of the couple acts in the same way as a housewife would (this is not true of
all gay couples, in fact it's very few but it was the case for
some), ie. He stayed at home during the day cleaning and socialising, not necesarily because he is gay, maybe the other person has a well paid job and it makes no sense for him to work as well when they don't need the money. There are a rising number of straight couples where the woman works and the man stays at home. The other works, has a well paid job and all the pairs assets are in his name because he paid for them. They have no children (obviously) and their families didn't like the fact that they were gay and have, essentially, disowned them. One day, tragically, the working man of the couple is hit by a bus on the way home. At this point, before gay marriage, the house and all the possesions of the couple would then pass on the the family who had previously wanted nothing to do with the pair, leaving the gay man still alive homeless with no money and nowhere to go. Now that gay marriage is legal the two can be legaly married and there are no problems arising from one of the couple being killed without leaving a will.