Boys and girls have lived and studied together since they were very young, and they don't know each other's gender emotionally.

The sentences after and are juxtaposed with the previous sentence, and the subjects of the sentences after and are girls and boys. The main part of the sentence is: (girls and boys) have no feelings underground. "Emotionally" is the prepositional attribute of "hypoplasia", which means "emotionally underdeveloped".

Therefore, it means "therefore" and can be regarded as a conjunction here;

In their relationship with the opposite sex, it means that the opposite sex has not fully developed.

So, the second half of this sentence is divided like this:

be

therefore

There is no emotional hypoplasia.

Relationship with the opposite sex.