Why are friends of the opposite sex taboo nowadays?

I am 30 yrs old and i am still friends with my friend group in high school except for one (Jay). My group has both guys and girls. Its been 11 yrs since i spoke with “Jay” and a photo of us came up on fb memories. I reached to Jay out since he is still friends with the guys, but he only seems to hang out with them when I/the other girls are not around. None of the other girls are in touch with him either. My intention was just strictly to be friendly and catch up since its been years. I never had feelings for Jay and never slept with him. There’s no sexual intention at all. Yes, i did sleep with all the guy friends in the group, but that was in the past and i dont do that anymore. I also never intended to leave his fiance out. I didn’t ask Jay to NOT bring his gf, but i never acknowledged him to bring her also. I assumed he would either bring her or shes ok with it like every other gf. All the guys in the group have gfs/married and there is no problem and i am their friends too. Jay did turn me down/declined politely and i thought nothing of it. Soon after i fb messaged him to hang, i got this angry (what i consider hurtful, inappropriate text from his fiance). I didn’t know all along i had my cell # on my fb page, immediately deleted that. You dont have to read it all, but if you skim through, its so hurtful and scary. Well that sucks I wasn’t expecting to get something like this.

Now it all makes sense as to why he doesn’t hang when the guys invite him when the girls are around.

Not necessarily looking for advice but what are people’s thoughts regarding on Why everyone is so fixated on gen italia? And why are people triggered when their partners have friends of the opposite sex nowadays? It’s been a few days i got this text, and i am still processing it. It’s very hurtful.