Most people don't know squat about hierarchy in the animal kingdom. Only nerds define themselves as Betas. In fact, they would be Omegas, bottom-feeders.
Basically, it's a dorky way to define people between the cool guys and the uncool guys.
A Beta is a follower that, in time, could overthrow an Alpha in a test. Alphas and Betas are the top of the crowd in animal hierarchy, and the Alpha primus inter pares. The Beta follows the Alpha but can always challenge the Alpha to a fight for the position of leader. Most times they lose and either die, surrender, or leave to found a new pack with themselves as leaders, but when they win, the Alpha is gone.
"Nice guys" as Betas? Gimme a break. They are the run of the mill packmates, they are ordinary dudes. As for nerds, they don't count. They are out of the mating game except with other nerdy girls, or when good girls get tired to be to used by us bad boys and need to settle down with a chump that will treat them like princesses, and nerds tend to make more money and be even more desperate so they are the ideal targets.