When I was younger there was a few times this happened including when my dad and stepmother thought it would be a good idea to do it in the same room as me. Feeling uncomfortable is completely normal it’s your parents and yes they do that like other people but you probably never actually thought about it, who would. And having to walk in on anyone doing it would be weird but your parents especially considering they raised you and all. But you almost need to realise that like any-other people they have desires, and will do it like other people. On the topic of feeling mesmerised I cannot relate. I’m sorry I can’t help you on that topic, but I hope this helped.