Is it okay to have sex while your in your period days? and what if the semen comes out inside the girl vagina?

Sexual Intercourse: Yes. Of course. Couples worldwide have sex during menstrual cycles. I am not sure how to answer your second question since that is the very definition of intercourse.

Well : It's ok to have sex while she is bleeding. Stds are still a possibility. Pregnancy is less likely but possible.

Yes: Intercourse during menstruation is not necessarily dangerous. Pregnancy is of course unlikely in this scenario. Use good hygiene and protection or birth-control still however if pregnancy is not desired.