Wait a minute...
In the chapter when they first had sex there wasn't any blood or discomfort on her end...
And before that her little sister mentioned something about her having some sort of romance related trauma...
Get the thought process, but, she does explicitly say in chapter 5 pg 3:
"So onee-chan was thinking of having
her first time with him"
The younger sister says in response:
"Are you dating him?"
And gyaru chick kinda just dodges the question saying "thats... well you know... not really"
And then the younger sister does say in response
"I know you've got some trauma but"
(she then just says that she needs to face it "properly" or both him and her will get hurt)
as for what that trauma actually is, idk, but I don't personally see any reason to believe there was some sort of sexual assault/ forced act or somthing. Gyaru even says she's a virgin and wants to have her first time with him so I'm inclined to belive that she's probably not gone through "that" kind of trauma.
Tbh it's not really that deep though and regardless of how many partners someone has had in the past, it could be a 100, or 10, or 1, or 0, it shouldn't have any bearing on a current relationship where the only thing that matters is that two people stay faithful/true and honest with one another, past commitment's are hardly relevant in real life relationships, and I think you can apply that to media as well, including this
 
Except when it comes to stds, that shits serious and not disclosing that when entering a relationship or just a fling is fucked. (Kinda just goes back to the being honest and open part though)