Of course no one should date or have sex with anyone they don't want to for whatever reason.
But the biphobia that's rampant on all of these threads isn't about that. It's about the vulgar, OTT, disgusted reactions to the idea of gay sex: "get out of dodge", "grim", more vomiting emojis than you can shake a stick at. Those are biphobic. That visceral disgust you feel at the idea of a man having sex with another man - that's biphobic and probably homophobic.
It is exactly the same energy as men talking about a woman who's had an adventurous sexual past as though she's been sullied or is somehow dirty. This person doesn't fit gender ideals (a pure, modest woman or an ultra-masculine man) - disgusting! Wouldn't touch them with a bargepole!
It also makes zero sense that a person's sexual history could actually alter your feelings towards them. If you love someone and are attracted to who they are right now, does it really make a difference if you find out that half a decade ago she had a threesome or he sucked a dick? Are we reducing ourselves/other people to our sex lives here?
Bear in mind that most people don't actually know their partner's sexual histories inside out. I'd bet more men have experimented with other men (before settling into otherwise straight lives) than any of you think: experimenting with other women is so much more socially acceptable for mostly-straight women, so most of us who've done it can be fairly open about it.
If you feel viscerally about your partner's potential bisexuality, you owe it to them to actually dig into that feeling and work out where it's coming from. And you owe it to society not to post in public about how revolted you are by it.
I would say that by the time someone's in their 40s they should have a decent self-awareness of their own sexuality, so the OP's partner should be able to be honest with her about what those experiments meant. If he's in denial about his sexuality, that right there is a red flag. If he's accepted that he's bisexual to a degree, and has awareness of what that degree is, that would be fine to me.