The topic of sex is very much present in the game, but in a more mature, subtle way, as has been mentioned (for instance, the miqo'te Path Companion is something of a nymph when it comes to your character - every sentence of dialogue she uttered was sexually tinged - I think there was even a line where she said something like "It's a pity I'm not a man you know, otherwise you'd know exactly how pleased I am to see you!"). Not that it was really offensive, just more for comic relief than anything.

As for marriage/child rearing in game, I only think such a concept could only really be viable if there was some kind of permadeath in the game (namely, your character dies and their offspring takes up their mantle and weapon, and so on, which would open up interesting possibilities for the game's direction, but ultimately lead to more problems than would be worth it).

It would also smack far too much of The Sims with a bit of Fable thrown in for good measure (not to mention the whole issue with younger players being exposed to such concepts and the resultant PR nightmare it could generate), so yeah, such a concept would not be desirable or even workable for FFXIV as has been stated.