How to stop guy friends from "converting" you

UPDATE: so after reading the replies I decided to message him asking if we could have a serious conversation about his behaviour, he agrees and comes round my flat and as I telling him how gross and inappropriate he was being, HE GETS IT OUT AND STARTS TOUCHING HIMSELF RIGHT IN FRONT OF ME. I quickly kicked him out and blocked him on everything

I just call him M, but basically we been best friends for over 20 years and he's never tried anything on with me and I never felt uncomfortable around him.....however recently he's been the exact opposite, ever since he find out my Ex was a trans women he's been constantly begging me to go out on a date, ignoring the fact that I already have a girlfriend ans that I have zero interest in men. Just this morning I woke up to him sending me a picture of himself...if you catch my meaning. And I just at the end of my teether because he really is my best friend and I really don't want to lose him but he's acting so disgusting recently I feel like I don't have a choice