I think just getting to know some transmasc people on a closer personal level would probably help dispel that. One of my dearest oldest friends is, and like, they didn't have femininity to destroy, that was never them, and it was literally killing them in direct proportion to how hard they tried to conform. So being with them through all that really eliminated any feeling I might otherwise have had along those lines.

Is it partly an attraction thing, do you think? I've felt a twinge of that for a different acquaintance, back when I thought I wasn't bi... if you are fundamentally just not into guys, a transmasc transitioning is going from someone you might have really been drawn to, to someone you're just not attracted to at all. But the person I thought was cute just like, does not exist, it was a facade, and even if they kept the facade up it could never have worked out, same as me and my cis ex. Not to say that's what it is for you, but it's worth exploring where these feelings come from if you want to attack the thoughts at their roots.

Another thing that helps me is realizing how shitty the gender roles are for both men and women. Both have huge downsides that nobody should feel bound by, and trans people especially suffer when crammed into those little boxes, on either side.

Ultimately I think time and exposure will help you work it out. Be conscious of how you feel and don't let it affect your actions, but there's no need to feel guilty, we all have biases and brainworms that we inherit from the world around us, and they don't just disappear as soon as we notice them, it takes time and effort.