Never have. Wish they would though but I guess expecting a mare to actually initiate is too much of a fantasy. (pardon my sour apples)
The problem seems to be that stallions are expected to initiate a first encounter and so they attempt to through a small show (usually through a one liner or some kind of joke because they are actually nervous as all heck). However, if this goes the wrong way (eg. the mare finds him unattractive or if she's just not in the mood) it can be classified as harassment and all manner of bad things and (worst case scenario) the stallion get's arrested/fired/etc. Due to this threat the "good guys" don't initiate as much any more because the risk is too big, meanwhile the mares are wondering why none of the good guys pay them any attention....
Of course, then you have the socially unlearned, slimes and such that don't known or don't care about the risks and they do it all the time....
Can't win. Maybe the mares should initiate? When was the last time you heard of a stallion filing a harassment charge against a mare?