A beta male is a male who buys into the feminist lie that men should be subservient to women. He believes that being submissive to a woman, caring, and gentle, will result in her returning his affection. He believes that he must buy women things in order to garner their attention. He does not understand that women desire confidence and willpower in a man.
Pete was a beta male because he bought the girl he just met at the bar a drink, making his subservience to her clear. He won't be getting laid tonight.