Well the way some people react to this whole M&J situation you'd think they were in an exclusive relationship because folks seem very deeply hurt by their apparent "betrayal" and lack of honesty even though it was, as you might say, just dating.
The problem here is the way the characters and events are written and how it relates to the choices of the MC. As it is written when playing the M&J route, none of these characters (MC, Maya, Josy) seem to be treating their respective relationships (MC/Maya, MC/Josy) as a casual fling given the way they talk about it and the way they interact with each other. All of them seem to be developing sincere feelings for each other and the MC even agonises over having to choose one over the other, which shows, at least the way it's written, that he sees a relationship with either one as something that he'd like to be exclusive.
However, where this becomes contradictory is in the MC's behaviour. For someone who seems to be very focused on M&J and a potential relationship with one of them, his "playing the field" makes his outburst over finding out about their relationship seem very hypocritical. He laments the potential loss of a relationship with one of them and feels "betrayed", yet he was going around fucking Sage, arranging a date with Jill, kissing Bella, letting Jade give him a handjob, and possibly even fucking Sarah, fucking Mona and Camila, fucking Envy, and fucking Quinn and Riona.
He may not have been in an exclusive relationship with Maya or Josy, but sleeping around with others while you're trying to start a potentially exclusive relationship with someone isn't something that "literally everyone who dates" does. Yes, he may be a young guy of 18 and this might be considered by some to be a normal thing at that age to sleep around, but I don't personally think it puts the MC in a good light to feel so aggrieved and hurt by Maya and Josy when he's been going after every pussy that comes his way.