Not enough eye rolls in the world… go back to the 50s you.
I don’t know OP. The thing is, sexuality isn’t easily defined - some people are v clearly gay, straight or bi, for others it’s a bit less clear. Either he’s basically straight, but open enough for the odd bit of experimentation in his youth, or he is bi, although possibly straight leaning. It’s worth pointing out that straight women experimenting are given a lot more leewa. and if it’s just the odd snog with a bloke I am not sure that is enough bi to qualify as lying to you.
I am not sure asking the friend he snogged is much help, because it sounds like he enjoys winding you up, and also there is a certain group of gay men who are convinced almost all men are at least bi.
I think you have to sit down with your fiancé in a neutral place, and say look I am sorry if I have come across as interrogating you, I didn’t mean to do that. But I would like to know if you are or have been sexuality attracted to men, if you’ve acted on that and if so broadly how. I know you aren’t obligated to give me any details, and all I want to do is get a broad sense of your sexual past, as that’s part of knowing who you are.
There is a risk he be seriously fucked off with you, but if it’s nothing you you need to talk about it, and much better calm and sober than drunk in a cab.
Then you just have to go on your gut instinct.