Depends on the situation. If you're in a happy loving relationship its easy to have girls who are just friends. If you're single, and looking for it, there's every chance you'll try and take things one step further, even if you're not physically attracted to her.
Personally as I'm single atm I must admit I've had a fling with pretty much all of my close girl friends (only under the influence though) - they all remain close friends.