Someone else
My girlfriend and I took a week off and I knew she was out with friends one night. The next morning after she went to work I stopped by to grab some stuff. I went to use the bathroom and I saw the pants she had on the night before at the bar. The bathroom smelled like her and I was getting interested. I grabbed her panties to sniff them and the crotch was soaked. I smelled it and it smelled like sex. I tasted it and yep, it was cum. The next day she said she needed time. The following day I went over again after she went to work. Different panties, same story. Another pair lay nearby and didn't look like that.
It's worse when it is your wife that leaves her panties out like that.
Cheating; move on before you get a disease. She isn't a girlfriend; she isn't a friend at all. Find a girl who doesn't go off alone with friends and who wants to include you in everything. Life is too short and there are too many single women out there who would love a faithful man to waste any more time on her. If she cheats when you are just going together, then trust me it will be worse if you keep staying or marry. Sooner or later she will cheat again. Once they get a taste of cheating it is all they want to do.