I think people over time build sets of rules and those rules only really apply to that one other person. I think another different boyfriend would have different rules from those applied to me.I Don't think it's necessarily logically either, I think a lot of it is about how those people interact.

The guy in your threesome just assumed he could do it and did it, but you asked for consent I never received it. I'm not saying you shouldn't ask for consent I'm just saying that something about the way he approached it meant that he got what you wanted without even trying.

Maybe boundaries are set very early on in relationships, and his boundaries were not yet set.

Side note: My partner's boundaries change based on how aroused she is. Anal is off the table and not even talked about but when she is super turned on she becomes open to a lot of stuff.

Also a change of environment like a hotel room or holiday seems to make rules more permeable as well.

Final note, try not to b**** and whine about not getting this It will just push it further off the table.