my niece has insisted that she's a trans boy because she hates having periods, avoids feminine things at all costs,

So she's like half of all women.

she legit asked me why I play female characters in games despite being male and not wanting to identify as female)

We used to make fun of kids way back when for being gay when they did this, most of this stopped after one legend responded that staring at a male ass was more gay. Then we said playing as male characters was gay. It was all nonsense and I'd not read too far into it.

I'm a bit of a cunt though, I'd probably say "pretty women are nice to look at, naturally you'll not have much experience with that".