So people start out as virgins – no diseases, no kids.
Then they start to have sex with each other, which can cause kids. They also have sex with the more experienced, which spreads the diseases to them.
As long as you can keep having sex with virgins and the clean ones, you’re good. The problem is you have no idea who the clean ones are unless you really get to know and trust them – and even then, if unwittingly they had sex with dirty partners, they might not themselves know it, even with STD tests (not all organisms can be tested for, and some also are not routinely screened, or able to be caught except on flareup). So only if you only had sex with people who only had sex with a traceable route of virgins, do you have any idea if your mate is clean.
You also have the issue, that every time you have sex with a new partner, they may get pregnant – either through contraceptive failure, or through malice (“forgetting” pills, even retrieving used condoms). If you are a male, and you don’t know your partner well, you have no idea if your female partner is trying to get pregnant.
The “easiest” problem is you catch a disease. Then you say, hell, I’m going to just screw everyone I can find and give it to them too. Of course, that’s what your other partners are saying, too – and that’s what causes everyone to spread everything. So you progress through HPVs and herpes, onto the (perma?) clap, syphilis, and other grosses, until you finally get to HIV, which will be a lifelong debilitating regimen. Do you really want to be disabled, and in pain?
The harder problem is you get kids, and those with women you don’t want to be with. We all could wish for the “accident” of having kids with a great spouse, but that is not what happens in more than half the cases of marriage, let alone non-marriage. Each kid you have is many thousands of dollars out of your pocket a year, plus dealing with the ex, and the child-rearing as well, which you probably weren’t going for.
Eventually, your sex spree will end with lifelong pain and bankruptcy, unless you are legitimately super-rich; you still have to deal with the pain, but it’s just a major hassle to deal with all the exes.
Some people talk about rubbering – that is, the use of condoms. Which doesn’t help the herpes and contact STDs much, and also leaves some residual risk e.g. when condoms break, of more serious issues. It definitely doesn’t resolve the malicious pregnancy issue unless you are immediately chucking the condom down the toilet or bathtub, not really a good plumbing practice. This is better, in several respects, but you are also just humping a plastic toy, and you may as well buy a real one that you can actually trust.