All the terms are really just semantics. But you should be able to take different girls out on dates without being being considered a cheater. Of course there are no "rules" so some girls might feel that's not right, so it's up to you to navigate that. But once you and your girl decide to take things to the next level, there should be an understanding that it's exclusive, if you start taking other girls on dates after that, well expect there to be repercussions!
That's pretty much the approach DPC has taken in this game. Only at the end of episode 6 is the MC starting to think he has serious feelings for Jill (if you go down that path), and a number of times he's commented to Sage (if you choose to) that he'd like more out of their relationship. The relationship with M&J is a complicated one, and they've decided to play it by ear, not lock it down or label it. And the relationship with Bella is just fucked.
The mc is definitely dating Jill (tennis, picnics) and by her own admission, they aren't yet in a serious relationship, and while she may be jealous of him dating other girls, they are not at a point where she feels he should be exclusive to her.
Having sex is no longer the ultimate defining point in a relationship, people hook up all the time, so it's ok for the mc to fuck as many girls as he wants, as long as he's not telling them that he's exclusive to them and that the relationship is anything more than fun.
In the end, each relationship is how you define it, and it's important that both people (or all three if you're managing that!) are on the same page, otherwise people get hurt. But feelings are weird, people get hurt anyway sometimes.