Ooookaaaayyy. While I've been hard at work, you guys have been tearing up the boards.
Don't faint anyone; but I will try to keep this short.
First, this is Japan and there is a culture of groping women there. Most of the women don't like it; but the key is for the guy not to get caught because the punishment is rather severe in terms of family shame, loss of income, jail time, etc. Sango's situation with Miroku reflects that a bit.
Second, this is the feudal era, where life was a little rougher than today. I have even read stories of entire orders of monks noted for being lechers. Miroku is no different than that. Plus he has a genuine motive, procreation before early death, or as Darwin put it, the evolutionary imperative.
Of course Sango doesn't like it. It is improper, out of place, and premature. That is why she thinks to herself that she wants him to touch the shoulder, or any place more romantic. She wants him to follow the protocols of proper courtship. From Miroku's behavior and association with Mushin, I suspect that he really doesn't know first hand how to properly approach a girl. If he were to court her in the customary fashion, and pay tribute to her emotions rather than her body as part of it, she would be more receptive. But Miroku is almost as bad in the romance department as Inuyasha, albeit in the opposite direction.
One of Miroku's comments when he thought he had finally connected with a girl only to learn it was a demon, was "why is it always like this?" The obvious conclusion is that his aggressive approach is a failure and he is slow on the uptake. Like almost all of the other characters in "Inuyasha", Miroku slowly learns and develops throughout the series. It is most obvious in the more consistent manga. After he proposes to Sango, he stops groping other girls and asking them to have his children, although he still flirts. Is this any different than a guy today dating other women?
Miroku does seem to respect marriage, though. He very quickly backs off when he learns that a woman is married.
As for his refusal to promise he would stop flirting, he was being honest. Until he actually gets married, and the contract is signed, he is still free to flirt; but he always treats Sango with more respect than any other woman he meets, except maybe Kagome. He refrains from groping her until she is well enough to fight back. He never asks her to have his child, until he actually proposes marriage. And he holds back, more so in the later episodes/chapters, from groping Sango when he senses she is feeling vulnerable. At those times, he does everything in his power to remove her emotional pain.
As for his larcenous streak, we see him at his very introduction to the story express disdain for a castle lord who grows rich and flaunts his wealth while his vassals live in poverty. He targets those people exclusively. When peasants who know him from the past speak of him, he is giving them his last bit of food to comfort them (e.g., Koharu). He has a sense of justice and is unscrupulous in enforcing it. (Thanks to LoverDemon for mentioning Robin Hood, an apt comparison.)
Miroku is loyal and reliable. He risked his life to help Inuyasha against Sesshomaru only a short time after they met; and helped to treat his wounds and shelter and protect him while he was healing. All this happened out of Kagome's presence and influence. So his devotion to Inuyasha, himself, can not be questioned.
As for his relationship with Sango, she was a demon slayer. She could and did handle him. In addition, she taught him in no uncertain terms what a woman will and will not welcome in a relationship. Because it flew in the face of both his obsession for leaving an heir and his compulsion to carouse, it took him a long time to learn the lesson. I also think there was a little element of foreplay there, take a liberty and suffer through the reaction, but at least get the reaction.
But he did learn it. He would rather leave Sango than harm her and vowed several times to run away if he thought his wind tunnel was losing control. He would rather die than see her injured; and despite his leanings, he gave her more respect than anyone else.
He was learned and far sighted, always trying to figure out what Naraku was up to before he could spring a surprise. And he purposely behaved the clown, sometimes provoking Sango on purpose. That was his way of coping with the death sentence that he lived under. When he was silly, it was on purpose; but his situation was deadly serious and much of his behavior was a reaction to it.
EDIT: So much for short.
But there are others 
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