The only problem I see is that with more rights come more demands. So if gays can marry, they will then lobby to marry within Church.
We then have conflicting rights. The right of the religious wishing to not allow gay marriage in church, and the right of gays wanting to marry in a church.
If the government forces Churches to allow gay marriage, they will then have to force Mosques - and that will lead to blood on the streets as we know our Muslim guests turn to violence with lightening quick speed when 'offended'.