It's a game: fans read into Garak and Bashir being lovers off-screen. I can see Troi and Data being friends-with-benefits. She enjoys his telepathic unreadability and sexual capabilities, and he finds her oh-so-foreign emotionalism and sensuality fascinating. It's not cheating on Riker for her because they weren't official during most of the series, and he's an android (doesn't count), and for Data she's a reminder of Tasha and the closest he ever been to love.
What else is going on just off-screen? Love-related or otherwise