“Why blame only the girls?”
Vertical transmission (mother-to-child) of infectious agents is well described for many infectious agents, by transplacental infection, passage of the infect through an infected birth canal, or by nursing.
Thus, it is a reasonable question.
Absolutely, however, the assumption of promiscuity of the woman is unwarranted.
One encounter can get you a trip to the STD clinic. A married woman who is totally monogamous can get it from her husband especially if he is sexually active outside the marriage.
Of course higher frequency and multiple partners increases the probability of all kinds of diseases.