@Muminabun
The church considers marriage to be between a man and a woman for the purposes of having and raising children. To be fair if you don’t believe in Catholicism why would you want a Catholic wedding. There is a debate to be had about the rights of gay people to get married but also the rights of faiths to exercise religious freedom.
Because the Bible (In Genesis) defines marriage as a union between man and woman, and the Church has stuck with that. It ditched a lot of other stuff, but kept that.
Once upon a time, the will of God (however it was interpreted) was sacrosanct, and effectively became the law, albeit depending on who chose to interpret God's word and how it best suited their own ends.
Now, the state determines the law, and even God (or at least his followers) are bound to abide by it.
Ultimately there's going to be a coming to a head. The Church has been pandered to for a long time, because it's been a significant threat to political systems and monarchies. For Christianity, perhaps that threat is very much diminished these days, making it harder for a Church to set itself apart from the law, or to gain special dispensations, particularly on equality. I think its days of special allowances are numbered, and it knows it.
Religious freedom can no longer be interpreted as 'as God wishes', instead, it's going to have to be 'as the law of the land wishes'. For too long the Church has been the harborer of bigotry, sexism, misogyny and worse, whilst wearing the cloak of liberalism and acceptance.