Why do some Christians claim gay marriage will ruin their straight loveless marriage?

Because they are gay themselves. They do not understand ity or uals and don't care to. This ignorance and intolerance is bread by conservative christians and other religions too (I live in the south, I mainly deal with the christians here) in order to cause problems and keep people distracted from real issues. Also, as they say, misery loves company. None of them get to be happy or do what they want in life so no one else should be able to either. Its pathetic really.