I strongly believe that there should be an opt out. First of all I don't think that having gay parents would be good for a child, gay unions are statistically less stable than marriages and the child is likely to face bullying when growing up. In most playgrounds, 'gay' is used as the primary form of insulting other kids. That's not right, but its a fact that it happens, and you would be placing the child under unnecessary bullying by providing them with gay parents.
But all that aside, gay people will be able to adopt from regular adoption agencies. I believe it is unfair to force the catholic adoption agencies to let gay people adopt. However PC you want to get about it, the act of homosexuality is a sin as written in the bible, which is indisputable. Now this is not to say that catholics have a right to hate gay people etc. the church takes the line 'hate the sin, love the sinner'. But you are forcing people to go against their religious beliefs by making them place a child in what they believe their God has said to be a sinful household. I believe this is going to lead to the closure of catholic adoption agencies, which is a sad thing as they provide a very valuable service.