I think they were going for a more "in a nutshell" approach—a shorter way of saying Kiro is very active, both sexually and physically, and being pregnant doesn't really suit her at all. I mean, she has a trophy room complete with pictures of all of her conquests, and she regularly updates that at the moment. She's happy in that lifestyle; I'd say if anything, becoming pregnant by Kiro would be more likely given her character. A no is a no, though.
Doesn't mean you can't enjoy the thought! You can write/draw/commission her however you want (I think?), it just won't be in the game.