If you make it clear that you are already have friends with benefits (or in fact just that you are only interested in that kind of arrangement), that that's your lifestyle choice and that holding to your values is the most important thing to you, "stealing" is not really an option.
I think you misunderstand. Whatever extra appeal you have to potential partners from having other people who "chose you (among others)" may be effectively negated because you're unwilling to abandon those others for the new partner. The people you attract probably don't want to share you.
So you and the person you were responding to can both be right. You're made "more attractive" but the "pool of women willing to date [you still] evaporates.
