According to the R.A.E. (Real Academia Española de la lengua), concubina hasn't masculine form. In Spanish, the only word cognate of concubina is concubinato. The word concubinato is used for refering to a relationship between a man and his concubine (English form for concubina)
I was making a social reference to the fact that men have concubines and women have affairs. There is a social acceptance for men to behave like this in many cultures, and that is evident in the words they use.