That's why I stress the importance of being sure of who you're with. Not everybody has an STD. I think it best to be with your partner for some time before having sex.
Whenever you sleep with a guy, you're exposing yourself to all the bad STD stuff that that guy was exposed to in his past sexual experiences. Many times, people have STDs for long periods of time without even knowing it. At some colleges, half of the women have chlamydia. Unless you're requiring your boyfriends to have complete medical checkups before engaging in sex (which somehow I doubt), it's only a matter of time before you end up with a bad surprise.
Not every STD has a cure. Some of them are your "friend" for life.