I guess that it depends on what people expect when reading fanfics. I don't care about fanfics, so maybe I'm not the one who should comment on this one. But I think that if I read them, I would expect them to stay true to the characters and storytelling from the video games. So I don't think I would want to read about Zelda characters having sex, personally.

To Casual Matt and Spearhead: What makes you say that the characters must be 20 years old? The age of consent in most places is 18. And if two characters get involved sexually, I don't think it would be very realistic for them to wait until they are 20 years old to have sex.
Again, not that I would want that in a Zelda fanfic.