Bisexuality Support Group

Sounds to me like your fiance is confused. There are a whole bunch of issues being jumbled together here and they really should be sorted through--by him. Balderdash- pointed out one element, the issue of commitment and monogamy. That's one issue that stands true whether it's men or women involved. It also is a part of the whole religious aspect of this thing. From the way you talk it sounds like you are practicing Christians, and many denominations of Christianity take issue with same sex circumstances. So there too, you have a commitment problem with the tenets of your religion.
I'm with Sapphire-on this. This guy sounds way to confused to make a decision. And I think of course you can stop him because it takes two people to marry. You have a say in this too. Why not continue loving each other and forget marriage until a bit clearer?
Is there someone pressuring the two of you to marry? Or perhaps he feels the need to marry to "prove" he's not gay?

Thank you so much mycolors you helped me in more ways than you can imagine.
I agree lucilu I think the more we try to suppress the natural the worse it will get. Believe me this is what I have been doing for a long time. Perhaps to long.
Thank you all for your words of wisdom. I have to get back to work again, but I'll be back soon. Take care everyone.

I want you to put the "infidelity" issue on the back burner, cause the issue is societal & religious shame which caused him to go undercover, but his honesty with you shows that's not the route he wants to go, only how he's been forced to.
Go to the library, & Barnes & Noble for books, & local listings for support groups, "cause there's a lot of work to be done here. And while I support you, don't feel guilty if it's too much for you. That's where his support of you steps in.

All very interesting and informative advice. I agree with almost everything that has been said here, and that isn't usually the case for me on these forums. I am sorry that you all are going through this. Tough stuff. And it sounds like the guy in this situation has some stuff to work out on his own.
I am going to use an analogy to talk about how the fidelity and sexuality are completely different issues. I am attracted to big black guys. They turn me on. There is nothing wrong with this; however, I am in a relationship with a white guy & have been for 2 years now. I am happy with him, pleased with the emotional side of things, fulfilled with the sex, etc. I don't sleep with big black guys because I am aroused by them. I very occasionally look at porn with them in it (this is acceptable for both me and my boyfriend), my blood pressure rises a little when I am around a big good-looking black guy, but I do not pursue them or feel like I have to sleep with them. And I don't feel as though I should purge myself of these feelings; there is nothing wrong with them. You could put women or asian chicks or any number of other groups in the place of black men, and the story would read exactly the same.
I think mycolours is right and he is not ready for a life-long commitment. If he is not done "trying stuff out", then he needs some space and time (outside of matrimony) to do so.
I wish you luck and compassion with your plight.