They were likely brought up to believe they should marry the opposite sex too, but that doesn’t make it good for gays.
Well qv if you don’t want to get married nobody will force it on you but you don’t need to deprive others of having that option.
As Sharon Ferguson says of civil partnerships; “they are still a separate system that was put together to stop gay and lesbian people from being able to marry.” …That’s the only reason they were invented.
It isn’t just a question of whether gay couples should marry- they already do. It is also a question of whether married gay couples deserve the same legal recognition as married straight couples. I don’t think there can be a sane argument against that.