you already know the answer by the sound of it.
but for kicks i was tempted to make the wincest argument, but after thinking about it i'm near entirely sure it's because of the player character is paul. when someone is attracted to a character they typically want them for 'themselves' in this case paul because we control his actions. (and paul in btd is pretty damn hallow making him an easy self insert if not his only use in the game
)
of course that could also be the other aspects could be the reason as well, such as paul peeping on his sister (those that insert as paul but dislike cest), or the more voyeur or cuck like elements with paul beating his meat while peeping.
though i would think the biggest issue is the lack of player agency in that part. you got no choice while viewing it, just viewing it or not. which is pretty jarring for a power fantasy. could've been very different with a [paul enters] and maybe also a [paul leaves] option.
(at least i don't remember there being a disengage option last i played) on a personal level it bugs me i can't boldly enter even if the result still ends up in no sex or sex acts. it would amuse me to irritate alia and maybe cause more sexual frustration so she might later accept sex with paul.