I have an ftm lesbian friend and he says it’s because he doesn’t fully identify with being a man, but that “man“ is the best-fit term for him. As in, he’s a man to himself and everyone else in every situation except for a relationship. He wants to be treated and be seen as something other than a man (closer to agender iirc) while interacting with his partner. I think that might involve some pronoun and gendered noun stuff but it’s more about dynamic.
There's a lot to talk about. Lesbianism has more than one aspect. Like other parts of LGBTQ+ culture, there's an entire community of support based around people with similar experiences, and lesbians specifically have a history of butch/fem stuff going on that historically overlaps heavily with trans men & transmasc people. There's also an implicit assumption here that the person passes or will pass, which isn’t always the case.
From what I’ve heard from he/him lesbians I know it’s that there’s a lot of baggage associated with being straight. Straight means not in the queer umbrella. Straight means loving in a straight way, which isn’t inherently harmful but also isn’t accurate to the way one might love. If that makes any sense…
To the people I know who identify this way, they were previously contributing to lesbian spaces and still approach dating in a queer way that doesn’t reproduce what's expected in heterosexual relationships. This, in combination with knowing that less than 2% of cishet women would date a trans person, while around 1/3 of lesbians would date a trans man, means "straight" trans men have a better, safer chance of finding a romantic relationship in lesbians spaces than in cishet ones.