Why is abortion not a valid method of contraception in the first place? What's bad about it, if not for the death of the foetus?
I think that most people would say you just answered your own question. The death of a fetus that would otherwise develop into a human is what's bad about it and what differentiates it from preventative methods of contraception.
If it is a valid method, then contraception is wholly effective at preventing childbirth, and it is not irresponsible to have sex without wanting children.
If we eliminate the moral implications of our actions, then a whole lot of effective means can be opened up. That isn't necessarily a good thing. We could dramatically decrease the cost of health care if we just killed anyone over the age of 65. They're all retired anyway, right? Off to the Carousel! Same with any handicapped person. Just kill them off. Think how much better the world would be!
By banning abortion, you could artificially create a set of circumstances that make it irresponsible to have sex without wanting a child. But 'it's irresponsible to have sex without wanting a child' would not justify bringing that state about.
And by banning the killing of anyone who is old or handicapped, we artificially create a set of circumstances that make it irresponsible to grow old, or take risks which might result in being disabled. I'm sorry, but the idea that we can make an irresponsible act appear responsible simply by not preventing the use of arguably immoral act to "clean up" the problem we created is just insane.
It's as nutty as saying we could eliminate theft by just legalizing it. If we just make it legal for people to steal stuff from others, then there's no crime if/when they do, so problem solved! Or we can just eliminate all private property and give everyone what they need, and then there will be nothing to steal. Oh wait! That's what some people actually do want to do, so maybe that's not such a great example. It's nutty anyway. You're not solving problems, you're creating worse ones.
You need an argument for why America is a better country if you change the law to induce conditions that make it irresponsible to have sex without wanting a child.
Um... Because America is a better country if our laws do place limits on behavior which a majority of the population believes is wrong? Until you can get 99% of the population to agree that abortion is perfectly moral right up to the moment of birth we must have laws which restrict to some degree the act of abortion. We can argue about where those limits should apply and how they should be applied, but there clearly must be some
This is not an all or nothing issue. It's a matter of degrees. Not everyone thinks that abortion should be banned entirely. Not everyone believes that abortion should be allowed up to the moment of birth. Thus, our laws and legal restrictions really should fall somewhere in between those end points. And as long as that is true, we are left with the issue of irresponsible sex you started with.