Churches are very much about belief (note, state churches should be excluded from everything I say here). If a church believes that homosexuality is a sinful way of being, not to mention having gay sex, then it makes no sense to ask them to sanction that behaviour in their rituals. Much like you would not expect a synagogue's kitchen to boil meat in milk.

More importantly. Any homosexual couple wishing to belong to particular branch of the religion represented by such a church would be expected to hold roughly the same views and would not want to get married to someone of the same sex and have happy, sinful sex sanctioned by ritual (yes, I am simplifying).

And as long as the religious ritual of the particular church does not have any effect on legal status and what you get to call it, that should not matter.