Firstly - just because a gay man had sex with a woman does not necessarily make him 'Bi'. There are a surprising number of gay men and lesbians who have 'done-the-deed' with members of the opposite sex.
Having been there (the falling in love bit with a gay best friend - not the sex bit), I am aware of how easy it is to fall in love with someone who gives you the things that you might not be getting from your DH.
In my case it was the fact that he was always genuinely interested in me, my feelings, what made me tick, my life, etc, etc. he always remembered the little things that I might have said in passing - but whilst DH often didnt pick up on the things that were really important my friend 'D' did. He was always there - no matter what (or who) was going on in his life. He could literally read me like a book, was extremely perseptive. We even developed some sort of psychic link ... We saw or spoke to each other every single day. If I was feeling upset or down he would somehow know and turn up on the doorstep saying he 'felt' that I needed him - even at one point returning from a holiday in New york when something major happened to me, even though I hadn't said anything to him, or enyone else about it(this also worked the other way too - I always knew when to contact him). The emotions invested in this friendship were incredibly intense. We did love each other and would tell each other regularly.
My DH was very jealous of my relationship with 'D', on the face of it, as they were both very important to me, they made a show of getting on face-to-face. In reality, both were jealous of each other.
IN the end, and with a lot of hindsight ... It literally came down to the fact that I got entirely different things from both of them. If I could have melded them together in some way - I would have had the perfect man. But it just can't happen. This relationship is unlikely to work out and you and he are not the only ones who could be hurt.
If you think your husband can deal with what happened - then tell him. If not, then you have to be the adult and deal with your mistake. Perhaps it was just one of those things. If you don't want to lose your husband and potentially your kids (or at least their trust) - then leave it, look at it as one of those things and get on with your life. This will eventually become easier to live with.