I agree April, have always thought Julie struggled with the obvious closeness of Brett and Jeremy, she may have felt abandoned by Jeremy and felt that 3 was a crowd which is a very difficult situation. Can understand how she may have felt angry and jealous.
That's not a feeling of emotional infidelity though which features people upset their loved ones turn to others for emotional comfort instead of turning to them.
It's more of a situation of her feeling this scumbag who her significant other was partying with day and night was making him act immaturely all the time and preventing him from spending time properly wooing her.
Oddly enough since I refused to party all the time with my party animal friends their girlfriends/fiance's would turn to me with their problems which included complaining about how immature they were and wanting to go party with their buds instead of spending more time alone with them. It cuts the other way too, men complaining about women who always want to party with their girlfriends instead of spending time alone with them.
there are only 2 things that happen when these conflicts happen-
1) they break up
2) the partier decides to give up the party lifestyle because they don't want to lose their love done.
Most of the time the former happens, far down the road partiers will finally grow up and realize what they lost and regret it.
A few girls I know are now in their 40s want to find a nice guy to settle down with but are having a hard time. For 2 decades they passed up the nice guys for wild funloving guys who they just messed around with. The nice guys either are already taken, got used to being single and like it and no longer want to marry or are at the stage where now they decided to try playing the field since they passed it up when young.
Jeremy didn't party enough when young for his tastes he killed his family so he could party every night. Julie didn't want that she wanted him to grow up. It is obvious why their relationship ended. The murders put a strain on their relationship not just because of the horrible nature of it, but because of the freedom to party all day and night that resulted.