No, there is no sex in this game (unless you want to count what happens to non-paying players when they try to play big against coiners :-P). I tried leaving Marauders and Bonnies alone on romantic islands, but the number of troops remained the same, so... no sex at all.
Any game with XXX content will have that explicitly mentioned (and most likely it'll be the game's strongest/only marketing point).
Jokes aside no there is no sexual content within the game. However in game chat can be inappropriate at times. Global chat is mostly dead so you don't have to worry about that. But if Brotherhood leaders aren't stopping it and go with the logic everyone is an adult, you may find times where you're not happy in Brotherhood chat. If your child is at an impressionable age I suggest playing with him/her rather than leaving them unmonitored.