I gotta say I don’t like the angle the author is playing with making toudou just a cheater.
They could have made it way lighter in tone by NOT putting toudou in an actual declared relationship, keeping up with the bit of “well it’s only polite”, addressing that she expects toudou to cheat, or just not bringing it up constantly.
It just adds a really gross layer of regret to what should just be funny/fan service that could be easily avoided.