Clearly someone that's never been in a relationship like that.
Seconded. It just happens. There's some chemistry, you're watching a movie or some shit together, you start cuddling, it proceeds from there.
Why not just ask her out? Asking someone to be in a physical relationship yet simultaneously be impervious to the pain of you getting involved with someone else is a lot to ask. And, in my opinion, incredibly unhealthy.
Get over your commitment issues and grow a pair of balls.