Pretty much everything you wrote is wrong.
Incest itself is not a trope, per se, it is just incest, and there are still many more games without it than with it, they just are not the ones people ask to be shared/pirated on F95. Purely for the purpose of illustrating the point, imagine a cissification VN/game where the grandfather is cissifying the grandson. It is certainly incest, and would include other popular themes/tags of many games including cissification, corruption, and male protag, but would it be a trope? Of course not. Incest in itself is NOT a trope. Mom-Son incest is kind of a stereotype, but we can blame Oedipus for that, and of course Freud for first pointing out it's stereotypical qualities.
To understand the ridiculousness and laziness of the shower scene, you have to actually think. What is the natural reaction to unexpectedly seeing a penis you didn't want to? How many sporting events where a streaker rushes across the field, sometimes televised to millions, end up with the streaker being inundated with proposals do you think? How many flashers get laid?
No, the natural reaction to seeing a penis outside of an already sexualized context is usually either laughter or revulsion. Don't believe me, go out into the street, walk up to a girl that isn't remotely interested in you and whip it out. Once you are released from the justice system, tell us how it went. Do you think there was any, however remote, spark of lust in her eyes?
If seeing a penis were in any way 'corrupting', to the extent of changing ones sexual beliefs and taboos, then every single guy who ever shared a communal shower or changing room in school sports would be gay or bi, right? Butt-fucked endlessly by all those poor corrupted humans who couldn't resist the corruption of seeing a dick they were not interested in. It is a stupid, stupid nonsense, and people *only* accept it for a moment because it is a trope. That is the laziness.