Because straight pairings are boring. Seen 'em all. Gay pairings (I like me some shoujo-ai/yuri too) bring new levels of drama into the mix. Coming to terms with being gay... coming out to friends and family... finding acceptance... It all just adds a whole new level of intensity to the mix.
Also, I was taught "fujoshi" applies strictly to girl fans. It's spelled with the kanji for "girl", after all. Think there's a male term too...