Right? My immediate thought was just that is is awful communication and everyone would have a better time if they just said something in either scenario
Oh and that is fine but then you have to ask yourself if you want to have a partnership with bad sex. If not, break up, or "teach" by communicating what you want, what is good and what is bad. There is no alternative, accept bad sex, break up, teach.
(Technically, you could let them have sex outside of the partnership to study, but... Well, not my cup of tea)