What an interesting premise... Dark yes but not overly so, unless you count the pregnancies... Well ok but there was no torture so it wasn't really that dark. Maybe a little psychological torture but really not that much, so Harry was convinced to have him self neutered, that's not bad. Every loving husband should... At some stage...
I just think that having a Slytherin kid is a classic and Trelawney not predicting getting wet when falling out of a boat is just perfect!