If he says he’s bisexual and wants to be married and you believe in a monogamous marriage then he’s willingly denying himself the right/ability to have sex with another man or woman.
Straight men who want monogamy are willing to deny themselves sex with al the beautiful, sexy women they come across. That may be less denial than a bisexual has to do, but it seems like a lot of self-denial to me. 
(Straight women who want monogamy are also willing to deny themselves sex with all the beautiful, sexy men they come across.) An emotionally healthy bisexual can be monogamous. (Of course, I have to believe that, given my W's choice of ap.)
The pain of being betrayed is awful. It's excruciating. It's almost unbearable. It brings immense anger, grief, fear, and shame with it. None of it is the BSes fault, but the BS has to deal with it.
It's normal to fear what life will be like if you stay with a man who cheated with men or a woman who cheated with women. You can't predict the future, and it's normal to fear the unknown. I bet you have fears about what your life will be like if you D, too, because it's unknown.
Now is a time to start processing the grief, fear, anger, and shame out of your body. A good IC can help. Your feelings are obstacles to deciding what you want. They're obstacles to seeing your WS(es) as they are - ready to change or distraught with regret, wanting to stay M without changing.
Forgiving, My heart goes out to your H. He was abused as a teenager, and he didn't deserve that. But he doesn't have a right to abuse you in turn.
My sense - from reading the pain that BSes express here - is that a same-gender A is about as painful as a straight A. Any A is awful. It's adds the sexuality question to the mix, but some gay M men cheat with women to prove to themselves they're not gay.
BSes need to decide what they really want (within the constraint of what's possible - you can't go back in time and prevent the A). If the BS wants R, the next step is to determine if the WS is a good enough candidate for R.
If you're both willing to do the work necessary to R, you can R. You can (re)build your M, and it can even be better than you ever imagined. I can attest to that. A number of SI members can attest to that.
R is not the only option. You can choose D with your head held high. As ChamomileTea wrote above, you don't owe your WS an opportunity to R. If you don't want to spend the rest of your life together, D can be great. If your WS is not a good candidate for R - that is, if you think your WS won't do the necessary work - D will be a lot better than R. A number of SI members have flourished after D.
Being betrayed is awful. It does destroy your life as you know it - but it also opens opportunities for you, some of which may be excellent.
[This message edited by sisoon at 12:21 PM, September 4th (Friday)]
