I can identify with that in some ways. There were times when I felt marriage was simply legalized prostitution. I just didn't have the same sex drive as my ex did. I think a lot of women feel this way.
Understand the being trapped - that's where I am. I know I would leave if I had earnings. The best thing I can do while trying to figure a way out for myself is to instill in my daughters that you should never not work - always have your own income. Marriage is just too disposable these days to be dependent on a man. As for the sex - sounds very degrading and that is a terrible way to live. I hope you -and I - can find a way out.
If you can't stand sex with him any longer, give him explicit permission to visit a prostitute. It sounds like you are just waiting for the right time to end your marriage. If his 'pressures' rise far enough, he may decide to end it before you are ready.