Teen sex

Why are people so against it? I'm a teenager myself, but in my opinion, I think it's normal to be sexually active with the person you're with. I know a lot of people who think that sex is strictly for marriage, or you should only sleep with one person in your lifetime (but, you never know, teen couples can work out) or something like that. I also know people who don't care if adults sleep around, but think that teens shouldn't have sex at all. Obviously, I can understand where theyre coming from in the sense that teens often don't know what they want, aren't educated on pregnancy/STD's, stuff like that. But if they're doing it safely and with proper protection (especially if they're only having sex with one person) then what's the problem? And if they're only doing things like oral and no penetration, and there's no risk of getting an STD, what's the problem with that? While most teenagers aren't prepared to have a baby, a lot of adults that have babies aren't either. I don't support having unsafe/unprotected sex, but I really don't support anyone doing that unless they're trying to get pregnant. Granted, mistakes do happen, but I don't see a problem with safe sex. Just an FYI, when I ask what the problem is I'm not being sarcastic or aggressive, I actually want genuine answers and opinions on all of this.