IMO just be your son's pop, not his friend. Meaning give him advice and counsel when he asks questions, but he doesn't need to know EVERY detail about how you get down!!lol
About not being stressed that you girl is bi, well, I was in a LTR relationship with a female who told me six months in that her 'roommate' was in reality her EX-GIRLFRIEND. And it wasn't some chummy 3's company bullshit 'cause he ex still had feelings for her.
If it's just sex, who gives a fucc?? But if you LOVE a bitch, competing with a dyke for some pussy is not cool.
Can't tell you how many times me and that dyke bitch almost came to blows!!!
If liking TGs is not a normal extension of your overall lifestyle, I would act accordingly and keep that attraction at a distance.
And stop thinking your son has a right to know about anything that you don't decide to tell him!!