The term is often used after discovering a partner has no intentions of dating you, or admitting to it, really. They will keep your hopes up, when in fact, you're just a leachy, horny mistake. The "slut" is probably fucking someone very close to you.
The slut is extremely attractive and thought to be the be-all, end-all as far as dating goes. The victim is often attractive as well, and has always ended relationships. So the first time it happens for the victim is when their feelings are the strongest.
Unless left untreated (keeping in contact with the slut), the coma can last up to 3 years. On one occasion I've seen it last to 5.
A "slut coma" can prevent confidence, an erection, social opportunities to meet new mates, and most notably, a hollow glazed-look for long periods of time.
Friend: Fuck man, you know what you're in?
Friend: A "slut coma," man. A fucking slut coma.