There are a number of reasons for marriage.

companionship

the intention to have children together

tradition

the desire to have a reliable sexual partner

the social expectations one has been brought up with

the thought that "everyone does this, and I'm a loser if I don't" (see above...social expectations)

the thought that it's a holy covenant (traditious religious veiwpoint)

legal and financial advantages

the view of the government regarding the subject (they can decide that you ARE married if you live together for awhile under the same roof)

Etc.

All of the above reasons can enter into the equation, as can others...but most of them are not obligatory, they're optional.

Most people do expect to have children when they marry, but a fair number do not, for one reason or another.

Gay marriage is no issue to me at all, one way or another, so I'm not opposed to it, and I have no problem with it being legal. Akenaton is correct to draw attention to health problems associated with the spread of AIDS, and doing so does not indicate that he's a bigot or that he hates gay men. I don't see, however, what it has to do with whether or not gay men should marry one another if they so desire. Churches shouldn't be pressured one way or another about whether they perform gay marriages, but should work it out amongst their own congregation in a democratic fashion...which many of them have done. After all, if you don't like this church...go to that church instead. There ARE options available.