Why? Tara was Willow's weird autistic friend until Faith turned her gay.* She wasn't written as a lesbian at first. To be fair, you wouldn't have to change much. Platonic girlfriends isn't that rare in college, although, I would probably introduce a third witch for season 5, mostly to make fun of charmed and give a better reason for why Willow can do high end magic.
*Faith turned you gay and Eliza Dusku forcing you into a gay awakening are two ways of saying the same thing.