What do you call a guy who is dating many girls at the same time?
a Don Juan; a ladies' man, a successful seducer of women, an obsessive and unscrupulous pursuer of women. From the legendary nobleman who seduced hundreds of women and was eventually damned for his immoral ways.
a womanizer; a promiscuous heterosexual man
a philanderer; a man who readily or frequently enters into casual sexual relationships with women.