My suggestion is that you both go to a counselor and talk about the situation, if you are thinking about accepting his proposal. Marriage is a major step, and I don't think I'm being cynical, but people can promise to change, but then once the knot is tied, they aren't so motivated. As you said, he was being clingy, and it is possible the proposal will be so he can latch on.

What if the sex doesn't get better? It sounds like a miserable experience for you right now.

What about the bisexual issues? Do you think you can stay happy just having sex with this one person, who is a man?

My final conclusion: Don't rush into anything.