I'm glad you made some sense of your situation and people helped you.
The term slut has virtually no positive relevance. A man or women who runs around offering sex can mean a variety of things. It can be the result of past sexual abuse, extreme low self esteem, some serious physical disease of the brain. People throwing the word around like they know a person, can be nails in an already abused persons coffin. Hence the reason I regret using it. (But if I explained you might understand why I did. No excuses though). Even calling someone an azzhole and liar is out of line, as those behaviors come from somewhere.
I do believe some people are capable of having a friends with benefits relationship though. Not me, but some. Special, free, open minded people with amazing, almost super human self esteem perhaps. I don't see anything wrong with it from a moral perspective or anything.
Too bad for your friend as I'm sure they are not happy about missing out, but I'm glad you came to a conclusion which aligns with your conscience.