Although I agree with you, many people who use your very same argument to validating so called "cheating" in a presumably monogamous relationship. I say presumably because, let's be clear here, societal constructs entail that we have only one mate, though historically and physiologically it differs widely between individual to individual. Sexual compatibility as a start to a relationship is quite common, but is most often recorded as a poor indicator of lasting chemistry. But by the same token, sexual compatibility or having sex with other people could have no correlation to that person's perceived idea of love.
Although an intimate experience for most, there are some indefinitely that are so outwardly expressive with their bodies that it wouldn't seem like a "special" occasion to have sex - likewise, saving one's body for another might also be rather inconsequential to these type of people, who deviate from the standard when it comes to monogamy.
I don't think that this story delved very deep into the notion of sexual activity as being ambiguous to romantic interest, but I do think that is what Reichi's character was going for. There are many tales I can think of that blatantly go for the reverse, where the POINT is that the two individuals clearly laid out their relationship in terms of monogamy, and the whole strife of the story was not dealing with sexual promiscuity as legitimate, but rather watching the suffering of the one who is cheating on, unconsciously enforcing monogamous stereotypes once more.
To me this relationship was SO one-sided! Getting “married” didn't mean anything, nothing, poof!, just air. Actions talk more than words, and the uke was too forgiving. Come on!! If they cheat on you once, what makes you think he wouldn't do it again, and be all “it was meaningless, so don't leave and don't overreact ”? For the love of– I've seen these so many times in yaoi and it always works to get my blood boiling and disliking the work. This mangaka is amazing, and I love their work but this one I won't let go so easily. Even if is fiction, some sense of respect and maybe making the seme work for the uke could've given this a nice twist. *sigh* Sorry, it's just that this one happened to be the drop that spilled the cup. Oh, just in case: yes sex can be an important part in a relationship, but it SHOULD NOT be the base for a relationship. Not told by me, but experts and personal experiences from others I've known.