I would think that if you are in a park or petsmart and both owners are there, you would both be liable if the girl got pregnant. It is both parties responsiblity to be able to have their dog under control.
If you were walking your unaltered male dog, and a female came well with a leash law, they would probably say that even if the female was loose, you should still be able to control your dog. Without a leash law the females owner has done nothing wrong I guess, neither has the male dog owner, so pretty much, it sucks for the females parents lol.
BUT with leash law or not, if your female is in your yard, tied up or fenced in and a male gets to her, the owners of that male dog are responsible.
Haaaa, I have been there, Hera was in our backyard, with 7' privacy fence, and a little mutt dog dug himself into her under the gates. I heard Hera and looked to see what was up, so thank goodness I cought the situation in time, so she didnt become pregnant.