@grace@weirder.earth in some branches of christianity (i am culturally catholic and it is strong there) there is a conception that only god is able to judge sinning i.e. it isn't the place of people to do so.

people who buy strongly into this idea might oppose, on theological grounds, such things as taking away rights from other people or otherwise demeaning them simply because they engaged in a “sinful” act, because that's humans trying to pass judgment which is rightly reserved for god.

more or less, that is what this statement is in reference to. it is saying “i personally think gayness is sinful, but i don't think it is my or anybody else's business to punish that, so if gay people want to go be gay then they should be able to, even though i disapprove and might tell them so”. it's like the old “i don't like what he says but i'll defend to the death his right to say it” only with sexual practices instead of free speech.

unlike with free speech, these people's tolerance for gayness usually evaporates once it starts happening in public.