I decided to have Sango change partners and move out because she couldn't work effectively with her emotions for him getting in the way and confusing her when she needed to stay clear-headed, and because she couldn't chance sleeping with him -- especially if he didn't love her back.
I had her say she wouldn't sleep with him until they were married because I think that's an effective way to handle Miroku. And that's really the way Sango handles him, as far as I can tell. She doesn't allow him liberties just because she loves him -- she understands that he's going to respect her body whether he likes it or not until she's ready to give it to him. That's why she gets mad when he gropes her. And I think that Sango really is a respectable girl -- one who would wait until her wedding night, even if her fiance's a lech. Especially if her fiance's a lech. That was how a woman got a Henry VIII -- refusing to sleep with him until he married her. Good ol' Henry was a lech too and went to extreme measures to be able to make her his wife. It's not just about purity -- it's about strategy.