/b/ - Siberia

I'm dealing with a real life situation that's made me re-evaluate how most people think about sex and 'rights' of sex. Basically: my friend of 13 years was seeing this girl (having casual sex with) for about 3 weeks, developed unreciprocated romantic feelings, and was then broken up with. They broke up in mid December. Around April he got the impression he might have a chance of getting back together with her and started bringing me with him to go see her and her friends. I got along with that group of friends far better than he and he decided to stop seeing them as much, said to me he didn't want to try and get back with this girl anymore, but made an offhand comment basically along the lines that he would be mad if anyone had sex with her (I thought this was a joke). A bit over a week ago me and this girl confessed to one another we had romantic inclinations towards one another. The day after we had (very good) sex. The day after that we both left the country and probably won't see one another for 3 months. My friend though found out we had sex and has decided he no longer wants to be friends anymore and is denying my requests to speak with him in person.

This whole situation has me thoroughly puzzled. On the one hand, I can understand someone being upset i.e. sad that someone they thought of romantically has moved on, and that those feelings might be compounded by it being a friend that was moved on to. On the other, I can't imagine being so angry that one would end a 13 year friendship over it. Either way though, it seems to me to imply a weird 'possessive' attitude over another human being such that sex is something someone has rights to and others do not. I find this attitude to be really gross honestly, it's a weird pathological objectification imo. To be fair, I am autistic and often misunderstand these value systems many people seem to hold dear. Can anyone help me understand the dynamics of what's going on here? My other friends seem to universally think I'm an asshole, with only two exceptions.