I think DPC put this dialog in to tell the players that they can continue seeing the other girls without risk.More often than not it means "it is a trial whether you choose to stay loyal to person(s) you claim to be deeply interested in, out of your own free will".
Just because he's told "no strings attached" doesn't mean showing oneself as a horndog who fucks everything that moves (plus Quinn) isn't potentially going to influence opinion about MC and willingness to continue relationship with him. Impressions can change, and so can opinions based on these impressions.
M and J are together, in a serious relationship. If we add the MC in the equation, we could wonder what was the status of their relationship.
So, other than maybe a few less RP if we do stuff with Quinn, we won't have a problem with M and J because of the other LIs, or the MC's hookups.