I'm probably gonna get called dumb for this but..

When a girl is called a whore or slut, it doesn't entirely mean she has sex with everyone or dates everyone. It also means that she might sleep with any boy who has a boyfriend and not care, and that makes her look desperate (like Lexi said). Also, a guy may call a girl a slut because maybe she cheated on him or only used him for sex. A guy wants a girl that he is proud of to show off to everyone. But what is there to be proud of when that girl has been with every guy? It's like saying "Hey guys look, I got the very first iPhone!" then someone else will say "Dude, thats old, everyone had that. Why are you so proud? Get one thats worth being happy with and worth showing off." So basically, the word whore or slut has a lot of different meanings. It could mean she sleeps with every guy. Dates every guy. Sleeps or dates any guy with a girlfriend. Only looks for sex within people. Cheats on her boyfriend. Steals her friends boyfriend or crush. You might not think it but being a slut IS something to be very ashamed of, whether you think so or not. So if you are a slut, a guy is gonna notice and not be interested in you. Because he's gonna think you're nothing special and you're just some girl who goes for every guy she sees.

i think getting called a slut is offensive because your sex and dating life is personal and the fact that you don't care who knows makes you look gross and desperate

If its not true, then it isn't offensive. Its just simply not true.
If its true, then its a fact. People view it as desperate and easy, so the person doesn't have much sex value. The person's sex activity should be his or her own private matter anyhow and is easy to spread rumors about a person's sex activity.
....But honestly, being offended has really no value either. Everybody gets offended at anything these days, nobody should care about it if it has absolutely no affect on the person whatsoever (besides that person's incompetence at controlling his or her reaction to simple words).

I'm not really sure why it would offend someone. I guess it would if they actually do sleep around. I've been called a slut and I don't sleep around, at all. I've even called my sister a whore as a joke, but it had offended her. I guess I really have no answer as to why it would offend someone as much. Especially if it weren't true.

We all have opinions, and if this is your opinion then I appreciate it. I like it when someone's not afraid to speak their mind :)