I have been called that (by someone on the EXs side) however I'm not sure if it's severe enough to warrant the label of "mistress" - that doesn't mean I don't feel bad for the way it happened.
Me and DP knew each other a few years before we became a couple, we both had quite strong feelings for each other (neither of us knew about) but due to many very complicated circumstances we both thought the other was "off limits" so we got on with our lives, I tried my best to let go of those feelings but they never seemed to fade like other crushes usually did over time.
Mine and DPs families would socialise with each other and one night at a family event we both had a few drinks, danced together and kissed. He had a girlfriend of 15months at that time who wasn't there.
The next day he went straight to the GFs house, apologised told her about it and ended things. She was obviously very angry with him and upset.
The day after he asked my DF(ather) for a drink and asked his permission to take me out on a date (he did it out of respect as due to families being close and a few other circumstances it was a bit of a delicate topic).
He then asked me out on a date and we've been together ever since (8 years and 2DCs now). He has always told me that he knew how he felt about me and what he wanted and he just regrets what happened with his ex as she was genuinely nice and he wishes he ended things earlier and more amicably and not the way it happened. But obviously he also thought there was no chance of anything happening between us due to the various complications.
However I have recently thought back to how one of the EXs family members called me a s**t and "his mistress" when we accidentally bumped into each other some time after it happened. It still makes me feel bad and sad and I wish things really happened differently but I don't know if I deserved to be called those things and just wanted to know what other people thought?
Me and DP love each other very much and we make each other happy, we both feel that we would have done things differently if we knew our feelings were mutual and I don't think we had an an affair, it was one kiss.
I am prepared to be told I am those things even though it makes me feel terrible even after all this time.
I just never asked anyones opinion on this before but it's always been at the back of my mind.