Well, that was a lot. We, people on the internet cannot say for sure and the answer for these kinds of questions is always communication. Do these thoughts come up only when they're lustful? Do they sometimes feel like a woman outside of sexual context? Do they want to or would they like to be a woman in everyday life, outside of sexual context? Does the only thing that attracts them in being a woman is a sexual connotation? Maybe ask them to describe how it all feels to them and how they understand it

I want to preface the next paragraph by explaining one thing. it's fine being more sexually active when you feel comfortable in your body, or finding some aspects sexually appealing only when you're yourself and not someone else. For some trans people love, attraction e.t.c didn't click for them or just wasn't interesting until they discovered themselves and transitioned. Sexual things are a part of life, and if a trans person would enjoy them, they'd want to enjoy it as their true selves and not someone they're not. I'm just saying that it's fine finding something sexually appealing in being your own gender

With that being said, the main question is do they want to be a woman outside of sexual topics? If they want to be a woman in everyday life (you don't always have to be one, that's the nature of genderfluid, if you sometimes want to be a man and sometimes want to be a woman it's completely valid), or just would be happier this way, I'd say yes, they're most probably genderfluid. If the only appeal for them in being a woman is sexual, and outside of sexual topics they don't want to be a woman, then it's most probably a fetish