Agawa is just being a coward. Like didn't one of her friends say Gotou should never sleep around with other women if he loved Agawa, but Agawa had every right to sleep around with different men?
Agawa didn't push back on that idea.
If I remember correctly the thing that Agawa's friend told him was something along the lines of "you shouldn't make Agawa sad" it was vague enough.
The "Agawa had every right to sleep around with different men" part was probably said by Agawa herself, but even if it wasn't its correct, why? because to this day Agawa and Gotou are just fuckbuddys (at best), sure it's clear that they like each other but they aren´t in a relationship so Agawa can sleep around but so does Gotou.
But yeah Agawa is a coward but honestly, in my interpretation of her character Agawa is constantly protecting herself from heartbreak, like Shiina ntr'd her with the boy she liked (however it isn't clear if Agawa was corresponded) and considering her reactions towards Akito and Kiyomi, she seems to have a problem with childhood friends of guys she likes, so to me all instances of Agawa being a coward are for the most part Agawa protecting herself sometimes mixed with a "Gotou is better without me" feeling.
But well this are my interpretations I could be wrong but at least i think they are educated guesses.