I've seen a couple of people attempting to justify their use by "If she's offended it's because she can't handle the truth/deep down feels that what she is doing is wrong."
Nope. While there may be specific cases of this, most people aren't offended by things because you've reached deep into their soul and found their crippling insecurity and just given them an epiphany. If I call you a jackass, the likelihood of you going "Hey.... maybe I'm a jackass." is pretty low I'm guessing. It's possible, but I think we'd be justified in assuming the reason the word is hurtful is not usually because of some deep down realisation. That's textbook victim blame. If I use words which imply judgement or distaste for someone, and they feel judged or that someone is disgusted by them, which offends them, that's on me, and I'm not doing them some kind of service by doing so. Humans are social, and we feel the impact of social pressures.
People are usually offended by the social meaning of the word, and that it's being applied to them. It's to do with how they value how people percieve them. Ever notice how if you annoy even slightly the roid-raging insecure overbearing types, they go nuts? Or if you try to insult slackers, it tends to be fruitless? It's because insults are primarily a social thing, and depend on the value that a person places on the language and those involved. Yes, I'm generalising. Deal with it, I'm sure you can read the rest into it.
Look at say, the gay community. They were told for a long period of time that they were wrong, that they were abominations, that they were damnable. Now, whether or not they believed that themselves, and indeed, some of them did, the sense of judgement and alienation from society was one of the bigger issues. People insulting them wasn't rekindling some long-denied truth, it was alienating them from society, which is where the hurt comes from. Most people value societal acceptance, and fear being hated or dismissed.
As several have pointed out, refer to a black person by the N-word. They're not going to treat it as if you've described their person. You've used a word which implies a history of inequality, and implies a distaste for them on your part. Deep down, they aren't going "Maybe I am just a N*****." They're going "This jackass thinks he's able to call me that?" It's the social context that matters to language, a social construct.
I think the real problem here is that you're trying to justify the use of the word in the absence of another word without negative connotations for people who enjoy promiscuity and sex for the sake of sex. We don't need to try to take the offensive word for this, we just make another. And for this, I fully endorse Abandon's suggestion, with Caffiene's amendment: Schwinning is sleeping around, and those who do, are Schwinners. And if they're doing it whilst on massive amounts of cocaine, they're bi-Schwinning!