I think Marl was like feeling to get all of MC.
I mean, if your GF behaving to be possessive of you and getting jealous being accompanied by someone close friends or colleagues.
The girls more likely are getting insecure of you, by having a large accomplishment, and they feel they dont help you at all.
Girls tend to think they will cast aside because of the possibilities other woman who are great than them will get close to MC.
But, Marl was already the GF of MC, and she already secured his V-CARD, there's no way a lonely heart from another world will just shoo away Marl just like that...
Although, Marl is a princess of another country, thinking like that politically, she had a reason after all...
BUT..............................
All im seeing was she was just getting jealous of all the attention MC just got....