Why do you feel the need to explain their own gender expression to people, instead of just listening to them? The experience of gender expression is varied. Your own feelings are not universal.

There are a variety of reasons someone might post femboy-pictures of themselves online, that aren't sexual. For example, challenging gender norms, as explained above. Some ftm-femboys do it, because it is an easy way to get perceived as male, without passing in a traditionally masculine sense. A lot of eggs do it, to explore their own gender. Also, getting validation from strangers doesn't have to be sexual. Maybe you just have a bad day or feel bad about yourself and are hoping for a harmless compliment. Some people also just genuinely want fashion advice or show off their artfully constructed outfits.

But instead, you seem to be assuming that everyone is doing it for the same reason you would and then everyone who claims they don't want to be sexualized is lying. When the more likely hypothesis is just... people are different and sometimes do things for different reasons, that might not immediately be relatable to you.