Since it was still early when I gave my answer, here's Part 2:
On the receiving end of the breeding spectrum, things look a bit less clear-cut, at least for me. While I still largely agree with our mouse overlord's points 1) and 3), it's much more selective and geared toward finding someone I -really- want to be bred by. Which somehow is harder than it should be because the NPCs I'd want to do that to me 1) don't have a dick or 2) can't breed no matter how hard you try.
The third category is NPCs that technically should be fun but I don't like putting up with them (has everything to do with attitudes and how, somehow, being on the receiving end doesn't feel nearly as fun for many scenes. It's all subjective though, again, and I'm sure others feel differently about it.
For example, I never could get into Dom! Sera and I skip initial Zephyr, but I genuinely love Sub! Sera becoming a dad and Zephyr mellowing the fuck out once she realizes she'll be a parent. It feels so weird that there's plenty scenes where Steele pays close attention to their partner and makes it pleasant but as soon as you want to receive, NPCs are egoists. Not all of them, of course, and some even have nice femdom scenes, but...yeah.
Which is to say, if William decided to, I don't know, write an adorable mousegirl with a dick who also likes to be a bottom, for instance, and is practically drowning in cute scenes? I'd put a ring on it.
Edit: The sadness I feel when I can't make Dzaan babies is real though.