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.