The raising of children by their biological parents is a good thing. Here's what follows.
1. The present legal definition of marriage is doing a **** job of convincing biological parents to raise their own children. Check.
2. The raising of children by adoptive parents rather than the foster care system is preferable. Check.
3. People, then, ought to be encouraged to get together and adopt children. Check.
Gay people ought to be allowed to marry, thereby creating a family structure that includes tax benefits, that would encourage them to start families of their own, for the sequential reasons enumerated above.
What negative affects would allowing homosexuals to marry have?
1. Allowing homosexuals to marry would in no way discourage heterosexuals from marrying.
2. Allowing homosexuals to marry would in no way discourage heterosexuals from raising their own children.
So...there are positive points to allowing homosexuals to marry. There are not negative points.