I've been rewatching the entire show with my mom and we've been discussing Sherry Dempsey.
Of all Martin's girlfriends Sherry was the most brash, loud and really kind of trashy.
Martin is a regular guy but he did have a taste for sophisticated and classy women. Hester Crane being the first he married. Even Ronnie while not sophisticated like Hester (operas, language, etc that she passed onto the boys) Ronnie was definitely a very classy lady.
But Sherry was just didn't seem like Martin's type.
Even in season 9-10 when Martin and Cam Winston's mom start hanging out you get why they click. She's Hester but black. Thus why Cam and Frasier are so similar. She doesn't look down on a regular guy like Martin and they're very opposites attract but have a lot in common.
Sherry was just the outlier that doesn't fit at all.