Yes thank you for bringing this up, that there is a difference between government mandated and religiously mandated marriage. Just a brief disclaimer, I am not against gay marriage/partnership nor am I religious, I bring this up however because my extremely religious friend (who is also very smart) has made this sensible argument to me before and I feel I must do it some justice by bringing it to light. Now the idea is that the church is a club and the club has a secret handshake. The club members have a strict rule agreement that the handshake can only be done with people who are right handed, they don't want left handed people in the club. Now the government passes a law that says that recognizes members of this club who do the handshake and creates laws that give the people benefits. Years later the government amends those laws to extend benefits to lefties so now members of this club can't exclude lefties from the handshake. Now the club members get pissed because they have to learn how to do the handshake with their other hand. Albeit the reality isn't that simple, the point is that the government shouldn't have been making laws that tell the club what to do in the first place. This is why Civil Unions are a good idea because it ignores the club and is only in the eyes of the law however the difference in benefits are still disputed and gay marriage is preferred.

Civil partnerships in the UK do have the same legal benefits of marriage.