In the case presented in the OP. No, the father should not be liable to pay child support. It was rape. I don't care if a 7 year old consents to sex. They legally can't. It's statutory rape.
If the mother never tells the father that she's expecting. Either from a one night stand or the relationship ended before she began to show and never disclosed the fact. Then no, the father shouldn't pay for back child support payments. Those can add up to thousands of dollars and the Child Support Agency doesn't care if you don't have the money. They'll garnish your wages and put you on a list at the post office and make you out to be a scum bag.
There are other situations that come to mind but would be hard to prove in court unless the woman admitted to it over text or there was a witness. Like purposefully breaking the condom/poking tiny holes in it.
You have to think that not every woman is a victim. There are many women who get impregnated by a man for the sole purpose of getting pregnant. These men are then forced to pay child support while never getting to see their kids. I've met several men this has happened to. That's not fair to the man to support a child he doesn't get any custody of. I'm also not in favor of men paying child support if they don't have at least part time custody unless the man in question is a felon or deemed otherwise dangerous.
