Ash has a female companion with him so there is a mixture of genders between the group.
They need a female figure to carry on the current plot, and if it were just Brock and Ash then people would be attacking everyone with yaoi. That's not saying much considering they already do...
The real reason Spoilers it's because ash charcoals their bikes with pikachu's electric attack well it's true *shrugs*
EDIT: Yeah... that too. It's some long running gag they've got goin' on.