Why? They're adults like your friend's gay brother, so what's the problem?
Whether kids who see the show end up gay or not has nothing to do with the show: sexual orientation isn't a choice, it's just what you are. If anything, the show telling kids that it's okay for two fictional 20-something women to fall in love is a good thing. Every little bit of acceptance in the media, especially family-oriented media, is a good thing: it's chipping away at the ridiculous idea that kids can't handle same-sex partnerships, when really, kids only react poorly to homosexuality if they've been taught to react poorly by bigoted adults who were themselves taught to hate. It's a terrible cycle, and anything that fights against it should be celebrated.
Besides, if you're worried that Korra and Asami holding hands and gazing lovingly into each-opthers' eyes is gonna get young girls imitating them, then you must be horrified about all the kissing the straight couples do - there's even a marriage: uh oh, all the little children are gonna be rushing towards the alter now.
The creators aren't pandering to the fans: they had the idea to maybe have them end up together even before the show aired. What the creators are doing is showing support for the gay community and trying to fight the exact sort of ignorance and intolerance currently dribbling out of your mouth.