Straight women using the term “girlfriend” to describe their platonic female friendships is not feminist; it’s not about “showing that women can be friends with each other.” It’s inherently lesbophobic. Built into the assumed platonic use of the word is the idea that women cannot have romantic/sexual relationships with each other. Straight women never use the term “boyfriend” to describe their male friends, because if they mentioned a “boyfriend” everyone would understand that word as referring to a significant other. 

If you’re a straight woman and use the term “girlfriend” to refer to a platonic friend of yours, you are by definition contributing to heteronormativity. You are actively making it difficult for people to accept relationships between women as serious and valid.