Your comparing this to sneaking "loving glances" at your mother while she was undressing was why I felt there was a sexual component in this.
I think your mother crossed several boundaries with you (sometimes it pains me to read what you were subjected to as a child). That may have let you to this kind of behaviour. Peeping through a crack at your undressing mother shows needs for being close to her mixed with misplaced sexuality.
"Taking in" sb (Ts included) while they are not looking is standard behaviour.
However, I still think you could benefit from talking about this (why and how you felt- which is anything but standard) with your T.
Maybe I'm not making myself clear here so let me clarify: What you did was normal (nothing to be ashamed of), how you felt (and what you were reminded about) is still nothing to be ashamed of but might be a good idea to work through it in therapy.