I have a serious problem with gay marriage, as marriage is a religious ceremony, so the state should stay out of it.
Sounds like you actually have a problem with state-sponsored marriage (gay or straight), but you're only willing to apply your logic to gay marriage.
Civil union is the state sponsored joining, and should be the proper avenue for the state to allow something that religion indicates is wrong.
Which religion are you referring to? There are plenty of religions that approve of gay marriage, and there are also plenty of religions that think various straight marriages (e.g. second marriages, or inter-racial or inter-religious marriages) are wrong. It's not clear why any of that is relevant to what the state should or shouldn't do, given that (in the USA anyway), church and state are meant to be kept separate from each other.
However, it has to be understood that most of the benefits of marriage have to do with holding a family together for the benefit of the children, which a homosexual marriage may have some issues in creating.
It's actually quite easy for a homosexual marriage to have children. Gays of either gender can adopt, and gay women can get pregnant and give birth. And if you want to play the "it's all about the children" card, you then have to explain why infertile/childless straight couples should be allowed to enjoy the benefits of marriage.