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 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!