If we ignore their suggestions that equal marriages undermine the foundation of marriage and marriage is for the procreation of children - marriages are undermined by factors like infidelity, abuse or contempt not by the institution no longer being an exclusive club for heterosexual couples, and couples who wed after the woman was fertile, who are unable to conceive and those who decide to remain child-free are also presumably wrong - we're left with the matter of choice.
Firstly the choice to be gay, lesbian or bisexual. If that's a choice then I and millions of others chose to be straight. I don't remember making that choice...because I didn't. Therefore why the assumption that people make a decision not to be heterosexual? It's clearly nonsense.
So then the assertion follows that if people don't choose to be gay they at least choose to act on it and here's where bigotry really rears its ugly head. What these people are doing is trying to deny mutually consenting gay people the same basic right we straight people have. The freedom and right to form sexual relationships. Instead they believe people should ignore their feelings and live an abstinent lie. Such arrogance (for that's what it is) is unbelievable. What gives anyone the right to take away somebody else's choice just because they don't like it?
If you are getting married soon then I wish you every happiness. The only effect equal marriage will have on my marriage is that finally I won't be in that exclusive club, I'm glad the doors are finally open to all. Now we just need to start calling it "marriage" regardless of the sex of the couple tying the knot.