I like my neighbor's 2008 Mercedes... I really, really like it... so I like it when they let me drive it. It's a wonderful feeling... but the car isn't mine, and probably never will be. But I really want it... so it's ok for me to steal it, right?
You like another girl's guy... you get to play a bit, talk with him... and he throws a little affection your way with flirting and such. He isn't your boyfriend, and probably never will be. But you really want him... so you're asking if it's ok to flirt and eventually seduce another girl's boyfriend?
See the similarities?
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen