Jealousy.
The first time I ever recall feeling (sexual/relationship) jealousy in my life was when MrS went home to go to a concert with his exGF/current FWB (code name SweetPea in my personal saga) shortly after we had had our "are we in a relationship" discussion (my first ever).
My stated stance: “Of COURSE you are going to have sex with her. You love her. You share all this history. Blah, blah, blah...” He says “you are going to be upset”. I protest. Short story long: he goes home, he sleeps with her, he comes back...I am upset. Not with him, but with myself for having these jealous feelings (I don't 'believe' in this "jealousy" crap...a fundamental view I have of myself at this time)).
We talk, I tell him that I am not mad at him but myself for not responding in the rational fashion I expect, he says “I told you so” - which drives me insane! So, a few weeks later SweetPea swings through for a visit – it's great! her+me, him+her+ me – so I figure out that I am not so much jealous because she and him had fun, but that she and him had fun while I was back at the apartment missing him.
I felt that I had come to a better place in understanding my own reaction and would have be fine with him being with her again without me there. Unfortunately I never had the opportunity to test my hypothesis, he never played with her again unless I was also there. “No, I learned my lesson.” Dammit!
So? Lesson? (for me at least) was that the jealously was situational...that the immediate object of jealousy ("You had sex with x-girl") was not necessarily the REAL underlying trigger...(finding the trigger, however, might be a sticking point...)
[PS. Don't...know...that...any...more...ellipses...are...possible...in...this...post!)