"keeping options open" = he isn't that into you...kind of like when you ladies tell us men that you just want to be "friends".
Steve hit the nail smack dab on the head. In my opinion, I wouldn't worry about why a guy does this or why a guy does that, because it gives me the feel that you're trying to fix him to be the man you're looking for. To put it in the words of a psychologist I like, "When you walk into a pet store and purchase a porcupine you'll walk out with a porcupine, it's not going to turn into a soft, cuddly puppy when you walk out the door." That's the way it is with men. When you find a player or commitment phobe, you'll have a player or commitment phobe, don't try to turn him into something he's not.......a committed man. If he's not willing to be mono a mono with you, then kick him to the curb and move on to the next. It's that simple. Not trying to sound harsh, just laying it on the line.
It seems that the OP asked WHY so many men seem to do the "keep your options open dance". She mentioned how a woman's fertility is limited and she asked why they do this knowing a woman will run out of the chance to procreate. Any comments on the actual question?