So you would never date someone that slept with someone else? 'Fuck buddy' doesn't imply that it's a one-way street and that one of them is essentially a 'blow-up doll' for the other one to use just to get off.
I suspect that the primary source of the confusion here is the word "was". pw0ncakes likely means that he wouldn't date someone who was currently someone else's fuck buddy. pyre is taking it to mean that he wouldn't date someone who had ever been someone else's fuck buddy.
Or I'm way off, in which case, on with your argument :)
The definition of the term 'fuck buddy' probably varies from person to person, so how exactly do you feel you can determine whether someone that you're dating/looking to date was someone else's 'fuck buddy?'
Interpretation: So you would never date someone that slept with someone else?
Am I missing a few steps here? I never remotely implied that.