I think I worded that wrong. It's not the fact that she's a sexual person that bothers me. It's the fact they have to make her continually sexual toward Sherlock.

Yes, because she knew it was the only way she knew she could rattle him. Remember that Moriarty told her his nickname for Sherlock is "The Virgin". So from their first meeting, she knew that was the one thing he probably did not expect and didn't know how to deal with. And once she realised that plan actually worked, she continued doing it. She was sexual towards Sherlock all through the episode because it kept him off his game. And it obviously worked, because he only realised she'd played him when Mycroft told him.