I can address that question/problem with our story.
Unlike you, my wife is bi and has no issue being with either sex and knows it's a turn on for me, whether it's another male or female.
We met a couple that are both bi and had them to dinner and a hot tub evening. The normal things happened. One fantasy of the new couple was to have their lady on the receiving end of a strap-on from my wife. No problem... I hadn't had the privilege of seeing that event before, but knew she (my wife) owned a strap-on. The next day, discussing how much fun it was, my wife said, "I'd like to see you with him". uh... uh oh... Realizing that my main objection was my own perceptions about two guys together, but in this scenario, everyone was bi, EXCEPT ME.
I told her I suppose it "could" happen, but I'm not sure how I'd even start. She suggested while we're all in a pile and she's giving him a BJ, I could fondle the other guy, see how I felt and play it by ear.
Fast forward two weeks and we're in the scene, hands everywhere, bodies intertwined, and my wife reached over and grabbed my hand and gently guided me to where she was, giving him a BJ. I reached out to feel his penis and realized it was almost indistinguishable from my own in size and shape, and in the dark, I could almost convince myself I was simply masturbating.
Her gentle urging continued, pulling my head down beside hers and then suddenly I feel his dick on my lips. I thought "oh what the hell, I'm the odd wheel in this group". and proceeded to play a bit, although not to completion, but certainly more than I would have ever guessed I would EVER do.
After that, things happened that caused a temporary rift in the relations between us and the other couple, although I believe it to be a temporary problem, we haven't seen that couple again, although we will again probably.
I've already decided "in for a dime, in for a dollar".
So, if/when we get together with them again, I have no doubt where it's going, and I'm both, a bit excited, and also thinking about the values (read that as JUDGEMENTS) I've had over the years.
With that said, after the deed, I may think, "ok, that's not for me", or I may think, "hmm, I wonder why I was so set in my own judgements that I've never even entertained the possibilities". I too have always been that "no way in hell" mode. Now it's more like "right time, right place, right people."
Go for it, enjoy and forget the demons in the back of your mind. Good luck.!