Yeah, I'd have to agree with you. If someone is sleeping around with a ton of different people to fill some hole within themselves, than I'd definitely say they DON'T know what they want. Someone who is in a committed, clean, monogamous relationship knows what they want..hence why they stick with it instead of running from man to man or woman to woman.
I don't think 'slut' can be defined by the number of people as much as by why/how they're sleeping around. If someone sleeps with a new person every week, but uses protection, gets frequently tested for STDs, and knows their actions are simply for sexual gratification...than okay.
If someone is sleeping with a new person every week and NOT using protection, acting ignorant about STDs and pregnancy, and thinks that every new guy/girl is the love of their life, or that their actions are perfectly okay...maybe a slut, and definitely lost. I think age matters too, there is a big difference between someone who sleeps around at 14 and someone who sleeps around at 24 or 34.