What do you call someone with multiple girlfriends?

Polyamorous people have multiple loving, intentional, and intimate relationships at the same time. Polyamory is a type of open or non-monogamous relationship that follows certain guidelines. Polyamory specifically refers to people who have multiple romantic relationships at the same time.

Is it illegal to have multiple girlfriends?

Having multiple non-marital partners, even if married to one, is legal in most U.S. jurisdictions; at most it constitutes grounds for divorce if the spouse is non-consenting, or feels that the interest in a further partner has destabilized the marriage.