I don't know that it's sexist as much as it is pointless. Back in the day, with smaller rings, smaller stadiums, and no electronics, it made a lot of sense to have someone displaying the round number, or a group of people to get the crowd into the game. These days, though, both ring girls and cheerleaders are really only there because they've always been there, rather than because they serve a real purpose.
That said, men want to see attractive women, and those women want to get paid because they're attractive. It's a business deal both ways, and not sexist as long as both parties are doing it willingly.