https://onesignal.com
It sends web/push messages to mobiles, Windows, Chrome etc - It is free for up to 30K subscribers and you can send unlimited messages - It is not for Android app push notifications though - That incurs a fee...
Might be a better option than the ad networks are offering push notifications and 30k is pretty generous.....
EDIT: I am using it on MainStream and I haven't checked on their opinion of Adult - Might be worth a look...