In the purest form of objectivity, sure. But then, in pure objectivity, I'd argue sexism doesn't exist, since it's a social construct.

It's making a generalization based off our society's residual stigma that most girls will/should become mothers. It also follows the trend of proclaiming that the actions of young people now will be the ones to foster the next generation, which is common for men as well. So, I don't believe it to be sexist, objectively or referentially. It's just one of those societal tropes that's persisted since time began. Go forth and have/don't have children!