I really don't know why so many posters are fixating on the bed-sharing. The friend is gay. It doesn't matter. The lines being crossed are all by the OP's partner.
It was made very clear from the get go that male friends are not an option. He’s always said that men and women can’t be friends.
Red flag. Utter bullshit. He's stupid as well as jealous.
Whenever there has been the remote suggestion of being friendly with a male colleague etc. even as part of a group, he’s been mad about it.
Wildly disproportionate reaction. Even if you don't want to be friends with other men, having good relations and socialising with male and female colleagues can often be important for career advancement.
We’ve tried to work on his jealousy over the years but it doesn’t seem to get any better. He doesn’t recognise that his jealousy is unreasonable.
It's not going to get better because he doesn't recognise it as a problem. He won't change because he doesn't understand that he should.
I am scared that the jealousy will turn into controlling behaviour once kids are involved etc
Please, listen to that inner voice. Your suspicion is correct. I'm sorry about this, and I can believe he has some good qualities - but these bad qualities outweigh whatever good there is, and they may become dangerous to you down the line.