Here's the best answer I an think of... When a woman spits, men could percieve it as a sign of rejection, i.e. the woman wants to get rid of his semen and is rejecting it from her body. Or, perhaps we as men, who view our semen as a valuable product to produce life and fulfill our biological need to reproduce, see this as a waste of our precious genetic blueprint. Note that these are just theoretical reasons as to why, subconsciously, men may feel this way. I think it's entirely instinctual and exists on an inner level, and there is no reason that men really know of (much like why we are so fascinated with breasts...).