I think that they didnt made a relationship clear even tho they were going out and having sex that doesnt mean Saachin was fully accepting at that point a gf relationship, its not nice or thoughful but I wouldnt call it cheating.

Ruki is so into her it doesnt seem to be a deal breaker.

The mangaka did an okay plot, characters arent both mad in love with each other, it happens irl