Actually, if this proposition was ever put to me, then the
answer would hinge on who the lady is. Before anyone jumps the gun,
I must stress I'm not for the idea of doing so simply for the
challenge of it, but rather only doing so if I genuinely believe
she's the one for me. 
That said, it is highly unlikely I will ever try this on a total
stranger simply because there is a host of issues to
consider(ie.character, compatibility, etc) and looks alone will not
tempt me to attempt hitting on someone(whether attached or not). In
essence, in all probability, I'd probably have to get to know her
well enough as a friend for me to do that if I decided she's the
right one.
I mean, if she possessed most of the qualities that I'd like in my
partner(in other words, women with her quality would be few and far
between), then obviously, I wouldn't hesitate courting her as I see
no reason to stay away from something which I could regret later on
- afterall, I only live once and I can't afford to give that many
opportunities a miss! 
Of course, I'd be more than willing to accept and
respect her decision whatever happens...
Personally, I don't know where those people who buy the "what goes
around comes around" logic are coming from - I mean, for me, you've
obviously got to be able to offer her something better than she
could have in her current relationship to convince her that she'd
be better off with you than her current flame. And if she was
genuinely convinced of having a brighter and more secure future
with me and that I could better satisfy her needs than her current
squeeze, isn't it only logical that she'd find it sensible that
she'd be better off with me? Afterall, everything in life and
love's a fair game and a gamble to some extent - even marriages are
not guaranteed to last a lifetime. So why should anyone hold back
from pursuing someone who is potentially his/her soulmate solely on
the basis of "what goes around comes around"? 
