The ONLY reason is the condition that their religion condemns homosexuality.
I won't waste my time trying to explain to anyone in here how homosexual parents don't cause detraction from the health or happiness of children; or how gay marriage would actually improve the economy a bit; or how gay marriage has nothing to do with polygamy; or how in any case you have no place judging polygamy, since other societies and cultures do practice without negative effect it and you just happen to live in one that does not; or how marriage began as a non-religious institution; or how I actually don't give a flying **** what your goddamn reason is, it's just ******** that straight couples get more legal benefits.
Not to mention the crap about STD's and sodomy. You do know these things called condoms exist, right? Or that sodomy isn't something that everyone believes will invoke Satan?
And promiscuity my ass, if you're getting married you're obviously not at the "let see which hot guys I can sleep with" stage, there's obviously a sense of commitment.