Seems like a cunt if she'd leave you because you're sterile.. there is always adoption... She's probably the type that wants to get pregnant by tricking the guy so she can trap them... I say "Hurray for sterile guys!"
PS: BTW... if you're sincerely thinking about pranking her about being sterile to see if she was telling the truth, is only giving off the image that you dont trust her enough to believe that she wouldnt do that... or you honestly think she will do it...I think you really need to analyze your relationship.. Shit happens, and people become sterile.. You should tell her if she becomes sterile, you'll leave her too.. Give he a dose of her own medicine