She was still a girl though.
We don’t know that. I mean, Milo’s former partner could identify as trans/non-binary, and that’s why Milo is using gender-neutral pronouns/relationship descriptors instead of saying “girlfriend.”
Additionally, more left-leaning individuals have taken on these descriptors to add ambiguity to gender in their relationships. It's non-judgemental and completely safe. Some people can still get offended by people saying “a guy and his husband,” and it can lead to unpleasant conversations. And some brittle listeners might not like “my wife and I” even in the traditional marriage sense because it's a Gender label.
Saying 'partner' avoids any labels of gender, marital status, sexuality, or anything like that.
Honestly, it's a good/hopeful sign that Milo might swing in the other direction, too.