Under-age sex/romance isn't allowed in video games, like in games that allow you to romance NPCs, children are just as immune to love as they are to war.
You can see this in Skyrim with the NPC Hroki in Markarth, who, despite being one of the more striking women in the game and an obvious choice for a romance option, is flagged as belonging to the child class, so won't respond to the Amulet of Mara and your interaction and conversation options with her are severely limited. I'd like to think this was intentional trolling by Bethesda rather than just a bug or database error, because she certainly doesn't appear to be an child and for many players, she is one of the most striking and noticeable NPCs you see when you enter Markarth.
I think the Fable series flirts with that taboo quite a funny way, like if you use a flirty expression on a child NPC, or even if a child gets caught in the crossfire of a flirty expression that is directed at an adult, the child will say something like "I don't like you any more", "But... but you're supposed to be a good person" or "That's just wrong".
Fable's recognition of that taboo, it's NPC's direct response to an attempt at romance on a child, combined with the way NPC's will follow you around town once your renown gets high enough, can lead to some unintentional and darkly humorous moments if you don't pay attention to your surroundings before engaging in sex with someone. Once I took a prostitute to bed with me and after the screen darkened and the sex scene played, when the image came back again, there was a crying child stood next to my bed, another child saying "You're a bad man" and an enraged parent admonishing me for my interactions with their child.
With all that being said, this is one taboo that I'm happy to have remain in videogames, as I can't imagine how it would be good or healthy for the industry to give the player that choice, even in the name of freedom and unparalleled interaction with the world.