That's a belief. There is no suggestion that anyone will stop him believing whatever he wantsA Christian baker may hold the belief that same-sex marriage is contrary to what is written in his holy book
Interestingly, it is reported that a case has occurred, in a foreign country, where such a baker refused to provide such a cake. I am not aware of such as case occurring in the UK so it would be interesting to know what might happen.
I heard of a case last year in France, where a shopkeeper put up a sign saying that Jews were unwelcome, during one of Israel's periodic bombardments of Gaza schools and hospitals. I believe it was said to be illegal to put up such a sign in France.