Question: How could the girls not see that Martha was a boy? In the movie his voice is deep well until Kim came to visit him.
Answer: Cross-dressing is a time honored tradition in comedy and it requires quite a bit of suspension of disbelief to enjoy. A character who is obviously a boy to the audience but none of the characters noticing is a big part of what makes these films funny.
Answer: Maybe they just thought he had a deep voice for a girl. I also thought about that, like when the girls hug Martha he's a guy - no boobs - but it's just a fun movie.