honestly even if u (royal u), as the reader, dont quite understand every characters decisions, which is definitely me sometimes, you can come up with your own interpretations that work. for example, with Jamie, you could argue he has a fear of being cheated on and so invites his buddy over to get his dick sucked or w/e so he can feel like he has control over it. maybe he felt particularly threatened by that friend? and then ofc Brit genuinely cheating on him just confirms his biggest fear & feeds into his deepest insecurities. even assholes do not like getting cheated on, lol. he may have genuinely felt he had a connection with her. add to that he's also turned on by being a "dominant" prick, more ego insecurities being fed, i.e. mistreating gabby, and it makes sense to me.
or hell, i dont know, that could be one of those stupid obnoxious "pranks" or w/e that some d-bags like to do lol "gotcha babe! haha!" there are some real dorks out there