Personally, I think you can gauge pretty acurately without actually having sex how powerful the physical aspect of a potential relationship will be. Especially if there is actual communication about sex beforehand. I can say with absolute certainty that I can tell how good a man will be in bed (with me) from the way he kisses (me).
Of course, a glitch does come to mind, that being the scenario from Sex and the City where Charlotte meets the man of her dreams who seems to be perfect for her in all ways only to find out that he is impotent the night before her wedding. It's a humorous scenario in the show, but one that would really make me question the honesty of my partner in real life - kind of making good sex/bad sex irrelevant.
So, yes the physical aspect of a relationship is almost (if not just as much as) important to me as the emotional, but I don't think you really have to sleep with a man to know whether he will be compatible with you in bed. Although, being the girl I am, I would sleep with him anyway if I felt that strongly about it. I'm not one to make rules nor to follow them.
