Sayako complains that MC has sex with her womb and that "That's wrong ... you don't use that place for sex".
Actually, gynecologists have found that women with partners who have a penis length in a certain interval is bad for them. That interval is where the penis enters and exits the womb with every thrust. This is bad for the woman. However, those who have a penis long enough to stay in the womb even when thrusting are fine in most cases.
Ergo, it's not actually correct that you shouldn't have sex with the womb, it's fine as long as the penis is long enough to stay there most of the time.
Not really relevant for me(you need like 9-12 inches depending on the woman), but it might be for some.