- There's nothing in the social contract of marriage that dictates the necessity of procreation.
- There's no upper age limit on marriage despite biological barriers to procreation.
- People can procreate just fine without needing to get married.
- People can live in a relationship that looks identical to marriage but without the formal registration.
- Homosexuals can and do have children.
For those who say marriage is about procreation, does that mean that anyone willing to get married has to a) show they are fertile and b) show that they intend to have children? Does it mean that once the children grow up that marriage can be dissolved without so much as an afterthought?
The argument is a complete non sequitur, it's just not relevant to the question. It's looking for a general biological justification as if an IS implies the OUGHT. The fact that homosexuals still have the ability to reproduce and that many heterosexual couples do not is unnecessarily complicating things.
This is nothing more than disguising prejudice. If as a society there's going to be a recognition of committed consenting monogamous relationships, then there's no reason to exclude on the basis of sexual preference and more than there is on appearance or ethnicity. I think that those who campaign against gay marriage realise this which is why they come up with such absurdities to justify their position.