I think it just shows how little they actually value women in any way, shape or form.
They see trans men as a total joke, that they'll never be a "real" man, and of course women would want to be a man! Being a man is the greatest thing ever and only thing these people have going for them, and in effect, transmen affirm their gender identity as being "the best".
It's the same reason they hate transwomen so much. They have such a low view of women, it disgusts them that anyone would willingly lower themselves to that level. It attacks their idea that being a straight white men is the best thing ever, and the idea that maybe it's not, and someone would willingly give up their masculinity and be happier is beyond horrifying to them.