I play lots of mixed in the winter and summer seasons. It is ALTA but it is equivalent of 4.0 to 4.5 USTA. I have no problem hitting at the female partner. I don't hit at her with the intent of hitting her as I will try to direct volleys to her feet or slight to her side. From the baseline, I will hit a hard topspin shot at her and from this distance, it could hit her if she is not quick enough to volley it. My female partners generally like me to play the women.
As long as you are not head hunting - intentionally trying to hit the female, I think it is fair play in competitive matches.