Part of marriage is to help keep genetic lines separate (avoid inbreeding). If a gay couple wants children, they would need to adopt (in case of men). The child's parentage might not even be known, or sealed. If they child doesn't learn to be like the parents, would need to get some genetic testing done, to avoid reproduction with a close relative. Most children of married couples, already know who they are related to, and know it's not okay, but some still do.
Thing about gay marriage, is that all the issues they claim to deserve, can be obtained through a lawyer, or the courts, without being married. But being legally married also gives them many things they can't use, or won't need, and that's why it shouldn't be granted. There is nothing stopping them from having the ceremony and celebration with their friends, changing their names, drawing up a will, power of attorney, adopting children, or most anything else a legally married couple can do. They don't get to file income taxes jointly (but many married couples don't either), they don't get Social Security benefits as a surviving spouse, maybe a few other things I can't think of at the moment, but none are a huge benefit.
The thing is, marriage is about a family, meaning raising kids, whether intentional, or just doing the right thing (avoiding 18 years of child support), and usually one spouse must take a lesser income earning role, to make time to take care of home and kids, and deserves something for that, it is work after all.