It might not sound fair, but I genuinely believe that it is the best of a series of difficult and unfair alternatives.
Firstly, sex is great, yeah. But if you're going to have sex, and you're fertile (i.e. haven't had a procedure to make it otherwise) then you always have to go into it assuming that there is a risk, however small, of conceiving a baby. That's just the risk you take when you have sex. If you don't use protection at all then you deserve everything you get. If you use condoms then less so, but still there's still a risk even when they're used properly.
So, now consider that something goes wrong and the woman does get pregnant. You might agree on what to do, either have the baby and keep it, have the baby and have it adopted, or get an abortion. If so, great.
But you cannot force a woman to have an abortion. It's inhumane. Especially if you don't find out in the first couple of weeks, because it then becomes an operation (and even before then it's very uncomfortable physically). You also have to consider the emotion of it. It's not just "take a pill and it's all fine". You are killing something that has the potential of becoming a child. Even if a woman didn't think she wanted children at that point, it becomes very different when she actually has a "baby" growing inside of her. Abortion isn't just something you go into and forget about. Okay so some women aren't affected by it, but others can feel guilty about it for years and years, if not forever. So it's a very difficult decision.
And if the partner decides he doesn't want a baby - then she has to go through pregnancy, childbirth and parenthood on her own. It's not as though woman get the easy options with it all. I would consider it a much easier option to send money every month and not have to have anything to do with actually looking after or raising the child.
Also, it's not a problem that only men have. If the parents come to an arrangement where the father looks after the child when it's born, then the mother would have to pay too, even if she chooses to have nothing to do with it. So that is by no means something that only a man has to deal with.
Ultimately, yes, it sucks. But unless you're going to start forcing abortion on women then you just have to realise that if you have sex, this might result in having a child, however small that possibility is. The only way to absolutely make sure you don't end up in this situation is not to have sex, and while this might not seem like the ideal option for you, then I'm sorry but sex isn't an absolute essential. You have to be grown up enough to realise the risks, and be prepared to deal with them when they occur.