(and by ditched, I mean abandoning someone alone at a club, bar, restaurant, anywhere)
I've had a few of those occasions where the guy who went with me to a gay club decided to leave with someone else. I'm a bit old school when it comes to that sort of thing, believing that "you should leave with the guy who brought you." That terminated the relationship right there.
One other time my BF who came there with me was flirting and carrying on with another fella. It passed the point of being innocent when he started groping the other guy on the dance floor and kissing him, in front of me and our friends who knew us both.
I simply waited until the dance ended, then very calmly & quietly told him I was leaving now, though we had made plans for later. I don't do big drama scenes in public, and even that much was kinda unusual for me, but you have to draw the line somewhere.
I had arrived there ahead of him on my motorcycle, he later in his car, so we both had wheels and he wasn't stranded (marooning someone is TOTALLY unacceptable, but I've seen that happen, too). He got the message, and never did that to me again, and we remained in a relationship, though a much less secure one, and eventually I left him for good.