I do not see it as a meaningless distinction. As i mentioned Mina not only is cool with the idea she is the one to propose it to Edwin, so she can better enjoy her previously repressed sexual desires. She is not interested in a (rebound) relationship right now.
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Meanwhile Edwin also knows he can't be in one because of his job.
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Maybe sometime more will come out of it in the future (or maybe not) but for now i don't see either side being mislead by keeping things casual.
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About Ian my hypothesis is that the main reason he got a girlfriend was because he thought this was one thing socially required of him. But he grew bored of keeping appearances after a while.
He makes fun of her when Mina talks about anything sexual so the one thing i am sure is that Ian was not trying to broad her horizons.