If you defeat a common naga type, you have the option to have normal sex, but she forces her impregnation on you
It forces a bad end. Impregnation is just a byproduct of it I believe. as in the condoms wouldnt mean anything since it is just a bad end. Also it wouldnt make sense how she tries to dominate you and doesnt take it off
it wasn't a bad end back when I played it.
if it does now, then it's a shame. there are plenty of ways to screw up her encounter anyway