I think that perhaps we'll get great porn games, great breeding games, but no decent breeding porn game; it's simply two things that don't combo well together, by the reason that somethingblue listed: assets.
If the breeding game is SFW you're probably breeding the critters for the gameplay impact; getting them to look different from their parents is nice, I guess, but only secondary. This means that there are lots of room for the developers to reuse assets. And even if your mino and catgirl fuck outside of the screen, and you get an egg with an old lady saying "I wonder how it appeared...", you'll still be happy.
However, for a porn game things are different. You want to see the critters fucking; and that means creating assets for every potential combination between them. For N species, you'll get N² combinations, this grows quadratically (and the game will be probably boring if there are too few species, so that N will be really large). And... sure, you can make the offspring indistinct from one of the parents, but that's also boring.