Now I'm just brainstorming on potential approaches:
Barring a DNA test (which is not reliable for inclusion - only exclusion)
If it was casual sex and there are multiple partners, how about some variation of market share liability for all of them?
Perhaps an intentional battery - exceeding the scope of consent by engaging in unprotected intercourse. It is a volitional act, intending offensive contact (exceeding the scope of consent), actual offensive contact results, without consent or privilege.
But for the guy's battery of the female, by engaging in unprotected sex against her wishes, she would not have have become pregnant incurring the cost of an ectopic pregnancy treatment.
DC
Ah, yes, DC. However, your approach assumes that the unprotected sex was against her wishes. However, in the specific case I'm researching here, the sex was both voluntary and consensual. It's unclear whether or not she was using any kind of birth control method.
If a woman wishes to lower the risk of pregnancy, she takes the proper precautions. However, as we all know, no birth control method is 100% effective except for .... drum roll .... ABSTINENCE (not having sex at all). By simply engaging in sexual activity on a voluntary basis, a woman knows (or reasonably SHOULD know) that it could result in pregnancy - wanted or unwanted - and she accepts the risk of that choice. That includes the long term consequences, including the medical costs of carrying the child, and the responsibility for caring for and raising that child once it's born.
Your arguments, DC, provide for the possibility of liability in a situation where the woman DOES NOT consent to sex. That is a perfectly good argument for the man to become financially liable for the consequences of an ILLEGAL act, which could be a pregnancy. I could definitely see the law making him pay for her pregnancy medical care whether she chose to carry the child to term, or if she chose to abort the pregnancy, because if it hadn't been for his battery, the pregnancy might not have occurred.