Every. Fucking. Time.
This stupid, lying bullshit:
MC: I come from a culture that despises sex, where all males must act terrified of mammary glands, and must fight as hard as possible to never have sex for our entire life!
Me: No, bruv; you're from Japan.
I mean... I've been there, and while I am sure that perhaps, oh... maybe 0.0000000001% of all males in that country may be like this? Maybe? If that? But, with that said... I am doubtful that 99.9999% of all Isekai Protags will have this much disgust, terror, and muppet-flailing incompetence when faced by a member of the opposite sex "en dishabille..."
It's just really shitty writing; it fails to be even marginally humorous, and just seems stupid as fuck.
And it is somehow an integral part of every fucking isekai?
You may think that every trope has a basis in reality, but this one Just. Fucking. Doesn't.