Twinkle Toes
Growing old disgracefully
The problem is that "civil partnership" is not marriage, it is set aside as being something special for gay people because they can't have marriage. It creates, for lack of a better term, a class system. Gay couples aren't married like straight people, just "civil partners". Allowing prejudices to persist because "their union isn't like a proper marriage".
Marriage is older than record. It isn't intrinsically linked to any religion, and each religion has their own rules and rituals. Legalising gay marriage is not interfering with religion, refusing to legalise it on religious grounds is religion interfering with law.
I'm not sure if that's your own considered opinion or you've copied it from elsewhere, but I think it's an excellent & succinct appraisal of why the current situation should be ammended.
I'd like to add that all couples should be able to have a Civil Partnership if they so wish. Equality means just that, regardless of the practical or personal problems that inevitably come about in its persuit. It's gonna be a long old process, mind...