When you have to think too hard about where to deposit your own semen. Or as I like to say: "When you really, really aren't sure where to put your slick little boys!".
"She told me she was on the pill, and yet I should have been wearing a condom anyway, and then she kept saying 'cum inside me!, cum inside me!'". It was quite the cumnundrum.
Example two: I started jerking off watching porn in my grandma's basement, and right before I was about to cum I ran into the bathroom for some toilet paper and I realized my grandfather was in there with diarhea. Dude, it was quite the cumnundrum.
Noun; A decision that precludes one from getting any pussy that night.
Dude 1: Man, I've got this awesome gig tonight across town, but my girlfriend is just getting back from a week-long business trip and is she going to be horny. I want to bang the hell out of her, but this gig could be our big break and if I go there's no way I'm getting any! What do I do?
Dude 1: I've got a final in the morning and the old lady wants to get it on. If I bang her brains out the way she likes it, I'll crash for sure and fail the test.
Dude 2: Last time I was faced with such a cuntnundrum, I banged my old lady and then begged for an extension.