Girlfriend hypothetically threatened me..how do I take this?

So, my girlfriend basically said...(in much more colorful language) that if I cheated on her (I never have or have given her any reason to think I did/will...she said this kinda casually in passing)...If I cheated on her she would cut off my penis...Which OK...I get cheating is bad bad BAD! but geez really? Cheating is not even close to a good reason for causing any bodily harm...let alone sexual mutilation. This comment has me totally up in arms. I never ever even though about cheating on my gf...not just because cheating is despicable and wrong...but simply Because I'm way too in love with my girlfriend to even lay eyes on another girl. But now, I don't know about that. I mean how would a girl react if her boyfriend said something "if you ever cheat I'll give you a good beating". And that's not even as bad of a threat in my opinion, I'd take a beat down as opposed to getting a body part chopped off any day. I haven't confronted her about it, I didn't act shocked or anything when she said it..but its been bothering me. Should I say something?
Girlfriend hypothetically threatened me..how do I take this?
Post Opinion