Simply, how would you react if one of Ash's traveling companions was a female-version of Brock?
Bianca's constant fawning over a man (or Pokemon) with muscles makes her gush and it seemed that she had an instant liking to Virgil. This was kind of a hint at what it would be like since she was a "recurring member" of Ash's group.
You know a female who is constantly falling for male trainers? I think it would be an interesting change of pace the same with Ash's main rival being a female trainer.