First off, the actual law. A man who is raped _is_ liable for child support. Women found guilty of statuatory rape have successfully sued for child support. If an anonymous sperm donor could be identified, that anonymous sperm donor can be sued for child support. Circumstances of conception are not relevant to child support law.
Now, how _should_ the law be? IMO, of course ...
First, no man or person of any sort should ever be able to force a woman to have an abortion.
Men, however, should be able to opt out of paternity, given that it is the woman's choice to give birth. Woman tells man she's pregnant, he has to say "yes" or "no" within a very short time frame. If he says "yes", he's the legal daddy, and he pays. If he says "no", he has zero claim of any sort on the child, but is not liable for anything.
Married couples would not use this rule, as marriage would be considered prior consent by the man to accept paternity. If a woman wants a guarantee ahead of time, she needs to marry the guy.