First off, I have no step-family, but I will somewhat defend this thread.
Adults should not be interested in the kids, but I can understand the other way around. Of course a boy going through puberty would be compelled to see a woman he is not really related to naked. Maybe sort of curious of what his dad dumped his mom for.
Also, a girl may be curious if the step-dad is well endowed. People are always inquisitive about penis size and the sex not being good could be a reason for the initial split. I know of one girl from school who purposefully walked in on her step-dad changing to see his genitalia. Lets just say she would call him "Todd the Rod" in private.