Transmen have been almost invisible in film and media coverage of trans issues until a few years ago, at most. On the few occasions transmasc people were depicted in films and TV in the past, it was almost exclusively from a gay or lesbian perspective, reflecting the growing visibility of transmen in those communities. The portrayal of such characters is very seldom flattering, and generally presents transmen as deluded tomboys or lesbians who can't accept their own sexuality. Outside of gay and lesbian culture, however, the media focus on trans issues has generally ignored transmen completely.
A big part of that, I suspect, is that transmen don't register to heterosexuals as a "problem" in the way transwomen do. The idea that they might represent a sexual threat to straight women is mostly irrelevant because all men represent a sexual threat to straight women and because heterosexuals tend to view penetration by a penis as the only authentic form of sex. Conversely, transmen are not a threat to straight men. Straight men aren't typically sexually attracted to transmen, and if they are it's because they see those transmen as women whose masculinity is a facade or pretence.
Think of the inevitable scene in every "woman dresses like a man to do man stuff" movie where the main character, rather than explaining the situation like a normal person, dramatically exposes her boobs to her love interest. That scene isn't upsetting to straight men in the same way the Crying Game scenario is, is it?
So yeah, beyond the lack of visibility, transwomen are percieved as threatening in a way transmen are not (outside of gay and lesbian communities, at least, and gay and lesbian communities still tend to be vastly more accepting than mainstream cishet culture). It's not because their identities are more secure. JK Rowling herself has wheeled out the lesbian feminist line that transmen are women who have been brainwashed by patriarchal ideology, but the consequences of believing that are still less serious than believing that transwomen are men who dress as women to try and sneak into women's bathrooms and trick straight men into wanting to bang them.