I only really like nullos if it's reversible. Dicks are just too damn fun to play with, and especially with animals that have a low level of secondary sex characteristics (and are a drawing...) it's nearly impossible to define it as a male without one.
If they have a featureless crotch that is still somehow extremely erogenous, to the point that they have orgasms just from the spot being rubbed, that's pretty hot, but that's extremely rare to find.