Well, let's see... here are the guys that SOME will say are out of bounds:
Co-workers, friends (former/future), family friends, a friends ex, a female friends bi-friend, former teachers, divorced, separated, to old, to closeted, to far away, to needy, to shallow, to fem, to butch. The list goes on and on.
Since you asked, here's the way I see it.
What two adults consent to do with each other is their business. If you discount every guy that someone else has a problem with, you'll be left with bathroom stalls and sleazy bars.
I'd rather take a chance an losing a friend than find out down the road that I missed out on the chance of life time. Trust me - it happens.
Co workers are just that - people you work with. Jobs can come and go but a life long mate doesn't walk around the corner every day. For christ sakes, the President of the U.S. is only only allowed to keep his job for 2 terms.
People will fire every pathetic, overused cliche at you they can. Don't worry about what isn't best for them but what is best for you.
Sometimes we need to take a chance and see what happens. It's better than kicking your ass for the rest of your life over never trying.
Thanks for listening.