Ah..alcohol, getting ugly men laid for ten thousand years....
Right, seriously.
I think it was a combination of the excess booze, frustration, and boredom. I don't think you are slutty, your friend maybe, I mean married, fucking one bloke behind her hubbies back,and then still going with this guy*, hmmm, maybe she is, but then <shrugs>. don't know her and her situation so maybe I shouldn't judge.
Regretting in the future? Sounds like you already are, which is plain stupid, just chalk it up to experience and move on. Cutting down, or even stopping the drinking, yeah, mabe that's a good idea if you regret what you do when you are drunk.
Sorry if this is a little rambling. waiting for the caffeine to kick in.
*I make off course a distinction of course between this and women who have sex with other men with their partners knowledge and approval. And who's hubbies are worthless and useless and so deserve their missus screwing behind their back.