I don't remember which movie it was in, but there were two men (filled with testosterone), and they had this bet that the loser would be the girl. It was never specified what gets to be the girl means. Is there a fixed meaning to it in English?
It amounts to derision, because the lowest form of life in the minds of little boys are little girls. My grandson, aged 9 next week is in the middle of it, and I don't think has invited a single girl to his birthday party.
But most well-adjusted males grow out of it by the time they are about 12. Others have it for life.
I believe the OP misheard the line quoted from the movie. Instead, the most likely and most common phrase said in movies where there are two men and a woman involved is the following
Loser gets the girl
There are two interpretations for this line. The first being that the two bikers are making a bet, and the loser "wins" the girl. The girl in question, especially in comedies, is normally overweight, unattractive, and/or extremely vapid. The winner of the bet gets to go out with the bestfriend of the former. And of course, she has all the typical characteristics of a beautiful desirable woman.
The second interpretation is also a clichè in American movies, where the protagonist is defined as being a loser; the man in question might be painfully shy, a nerd, a bit weird looking, an outsider, someone who is easily pushed around etc. At the end of the movie he and not his best friend or nemesis gets the girl.
TV tropes has this to say:
Loser Gets the Girl
Two men get in a fight, or at least a highly charged physical confrontation, over a woman they're both attracted to. One of them wins, so naturally she spurns him and goes for the loser
Ten films where the loser gets the girl
... yet another movie where the schlub gets the hot girl has entered the history books. It’s a trend that seems to be gaining steam lately, but it certainly isn’t anything new.