I can't accept Cooper as a sexual being.

I don't think there ever was anything between Diane and Cooper other than friendship. I think the original kiss (Diane said they only kissed once before) was a way of telling the difference between real and Tulpa/Doppelgänger. They have to kiss to make sure they know it is the real one. When they meet in the sherrif's dpt and in the hotel because Diane was out of Coopers sight (and there was a Diane double there so they are aware of the dangers of other thems still being in play).

Coop and Diane seem to have had a plan which we only saw a glimpse of (he asks if she remembers everything) and it involved crossing to the other dimension by having sex. Obviously this was painful for Diane, having been raped by Bad Coop, and also shows why Dale shows no interest or emotion during it. It must have been too much for Diane, who after crossing, becomes Linda and forgets, whereas Cooper remembers he is not Richard.

I can't accept Cooper pointing a gun at innocent people either. There's no way it was really him in episode 18.

Actually, while mostly morally upright, Cooper seemed to be a flirtatious ladies man. He had an affair with Caroline in Pittsburgh. He had instant chemistry with Audrey and later conceded that he desired her. He fell in love with Annie at first sight. He was even kissing the "beautiful woman" Laura in his dreams. I'm not surprised at all that he had kissed Diane once before.

All that said, the sudden romance between him and Diane felt forced. I don't think he was in a romantic relationship with Diane. She worked for him and they were friends. I would have preferred Annie or Audrey be his love interest in S3.

He is a dead fish in bed! As dougie with hot af janey E, he lay there like a dead fish while she did all the work. But at least he was smiling then.
As himself? with Diane, he lay there like a dead fish while Diane did all the work. He didn't smile, his face never changed. No wonder Diane was so freaked out she had to cover up his face with her hands.