I was living with my first partner, and my ex was coming into town on a midnight flight. His elderly parents were gonna drive 65 miles into town to bring him back to their place for a visit. But at the last minute his mother said it would be very difficult for her at night, especially with her husband having early symptoms of Alzheimer's (which eventually killed him), and asked could he stay at a local hotel until morning.
When he told me this over the phone earlier in the day I asked my partner if he could stay in our spare bedroom instead, spare him the hotel hassle & expense. They had met before. I'd pick him up at the airport that night, and even drive him out to his parents the next day and save them the trip altogether. Plus I knew and liked them, a chance for me to see them again.
When I returned from the airport after 1 AM my partner was already asleep in our bedroom. I helped my ex settle in, and as I asked him a last time if he needed anything else, before retiring to my own bed, he called me in.
He closed the door behind me, and started hugging and kissing me, and pulling me down onto his guest bed. I wouldn't return his kiss, and said I was married now (not legally) and monogamous. He didn't want to take no for an answer, countering that my partner was asleep and wouldn't be the wiser.
You know, I wasn't the least bit tempted. As if I'd ever betray my partner under any circumstance, but it was especially reprehensible to me that my ex would try to seduce me with my partner sleeping just 20 feet from us.
I broke free of his embrace and got into bed with my partner where I belonged. Next morning I still drove him out to his parents like I had promised. But he really lost my respect after that, and I tried to never be alone with him again.
Didn't tell my partner about that incident, though. My ex visited his parents a couple of times of year, and the 2 of them would cross paths at the local gay nightclub. My partner was very possessive, with a quick temper, and I wanted to avoid an ugly and possibly violent public scene. I hadn't cheated on him, nor did I ever in thought or deed, so there was nothing for me to "confess", better to just keep it to myself.