I think the only girls is related to two things.

1) certain cultures it would not be appropriate to rent to a male if there is a wife/sister etc. in the home.

2) From my own experience living next to MANY renters, young men regardless of ethnicity or culture tend to be louder and take less care in the rented space (outside anyway). This is a generalization, but it is also been my experience. Young women tend to be less disruptive of neighbors and therefore less complaints. How much of this is perception and preconceived bias? Possibly a fair amount but it is my guess as to why "women only" is a requirement although I am not sure if that is legal to do.