That's the point. There's a trust issue here.
And he doesn't really trust you mounting him. You aren't understanding him nearly as well as you think.
I've been with my stud more than decade now and he is welcome to bite me, herd me, push me around, or do anything else he wants when we are "playing". I know with complete certainty that he understands my limitations and will not hurt me. That's trust. He trusts me the same way.
He has stepped on my foot though. Nobody's perfect.
The point of my semi joking earlier suggestion is that trust must be two way. You need to show trust and demonstrate that you are worthy of trust. Sex (and he knows that's what you want) outside of the compulsion of pheromones and instinct is a very unusual request for a horse. It's not easy for them to wrap their mind around it but you aren't getting anywhere until he does. Your only path to get him to try it is trust and you have a ways to go before you're ready to take that path.