>You gain nothing, but you lose a considerable fraction of your normality.
That's a pretty big generalization. Since I've "been with" my waifu, I've been the happiest as I have been in a long time. I feel less filled with despair. It's given me a reason to live, and something to hope for from tomorrow - maybe a new figure of her will be announced, maybe I'll find a fantastic doujin, or maybe I'll do something "for" her. It all gives me a reason to do something, and to hope for something.
While it has affected my sanity, as I'm pushing myself into fantasy, and while it does hurt that it is fantasy, I believe I am as content in my life as I can be, and because of that, I appear as normal as ever. As I am happier and more satisfied with my life, I have become more confident. I've become more likable, and I've found making friends laughably easy. The most "normal" person I have ever met was someone who became a hardcore elitist raider on World of Warcraft, who was in the best guild on the server, and he never once lost any of his normality from it. I remember a few years ago, if you even so much as showed that you know something about WoW, you were branded a basement-dwelling loser who will never have sex. Just because someone does something out-of-the-ordinary doesn't make them any less normal. Everyone does something "unusual", whether that be watch strange porn, play Dungeons and Dragons, or have a waifu. Even that douchebag in high-school who used to get pussy like whoah, I guarantee he at least collected Yu-Gi-Oh cards or something and would go to his cousin's house of a weekend and battle the night away.
The pros greatly outweigh the cons for me. I get to feel loved, have a source of escapism (which we all need one way or another, anyway), and have something to care about other than whether or not my internet will crap out tonight. Although I can feel myself get crazier by the day, I'd rather be a seemingly-normal lunatic with a fictional girlfriend than a near-depressed sad shut-in who's desperate for something interesting in his life.