I have had a few relationships with bi females, one even let me watch though I wasn't ever in the room, and didn't participate in any way. It was poetry in motion, and something that I will always remember as a beautiful moment in my life. I have never revealed to friends that it happened, and would never reveal her secret to anyone, even though we parted many years ago.
Any male who believes that simply because they don't agree with something, then it won't happen, is deluding themselves. If a female wants the experience then she will go and get it, with or without approval. Better to embrace a partner for who they are, and who they always have been, than live in a world of denial.