For anyone who pays attention to Japanese 18+ games (such as this one), it's always been "obvious" that Android would soon sport 18+ games. Of course, like some "obvious" things, it's one thing to make a prediction, yet another thing for it to become reality. Especially if you're looking for a serious attempt.
I only just found out about this, but VisualArt's set up an Android store [WARNING: 18+ content] late last year that opened earlier this year. Beyond their own products, they also have Android games ported from Windows from a couple of other notable manufacturers (130cm, riffraff, ZERO). Naturally, this approach of a third-party market isn't likely to get very far on iOS due to the requirement of jailbreaking on that operating system, something that's a bit hard to convince users to do.
It'll be interesting to see if this approach actually leads anywhere!