Even if you don't hit the mark on the first try, you just keep at it until she gets pregnant.
Literal
One-shot [with-で] hit not-even-if, pregnant become [until-まで] keep-at-it-[conditional-ければ] only-need [besides-し].
一発で命中 'hitting the mark in one shot' uses a military/archery metaphor. ハメ続ける is crude slang — ハメる is vulgar for sexual intercourse, 続ける 'to continue.' The だけだし 'it's just a matter of, and besides' is dismissive. Extremely blunt and crude.