Lena is very protective over Holly and doesn't believe that she can take it being used by man, hence she either drops Ian or leaves him so that he and Holly get together, if he fucked Holly. Though it is possible that it ends in a throuple in which all threee love each other.
A similar case is when Minerva ends her affair with Ian if he dates Holly, because she believes Holly wouldn't take it well. Once again, Holly is the only girl where Minerva cares.
Louise also doesn't sent a text if Ian is with Holly. He can be together with every other girl and she still texts him to flirt with him. This could be sloppy coding to avoid messing with Holly, or Louise knows Ian is taking with her and once again doesn't want to interfer, while she has less issues contacting him if he is with Lena (currently a questionable plot point, at least Ian comments on it and can back out because of that).
Overall, its impossible for Ian to cheat on Holly in the long run, he automatically ends their relationship on his own if he still has feelings for other people, because he doesn't believe he can make her happy, then.
Hollys special threatment is sort of consistent in that people around her correctly assume that Holly can't take getting cheated on as well as say Lena, who is a used to it and tougher. Still, EvaKiss bias and special plans toward Holly are noticable and it can understandibly seen as inconsisten since Ian and Lena are behaving different with other people.
While Ian and Lena can cheat a lot, at least in Ian case it starts to have consequences and he is forced to decide between ending love relationships or stop cheating with some exceptions. In Lenas case, it is questionable how she can still fuck 10+ people in one run and don't commit to anyone aside Ian. Supposedly its the nature of her relationship that she isn't serious with most of them, but she really should start commiting and ending other relationships. She does it with Ian, but there are still many flings that can still be go on and it can be seen as a writing inconsisty, either voluntarily by Eva, or involuntarily if the many choices and paths just overwhelmed them.
Do I have to remind you that Ian knew Holly before either of them knew Lena and that Holly wanted to sleep with Ian? Holly invited him to go with her, and Holly "messed up" their sleeping arrangements, and it was Holly that kissed Ian FIRST!
May I also remind you that Lena is so protective of Holly, she lets her sleep with Ivy and encourages her to sleep around with other guys that Ivy suggests... some protection.
Ian actually cares about Holly as more than a piece of meat as they have the same interests and... most importantly, IAN AND LENA WERE NOT A COUPLE AT ALL!
Shortly after all that bullshit happened, Lena calls up Mike, who has a girlfriend, and has him over to her place and fucks him. The next morning Mike left early and Lena finds out that Jeremy, her best friends boyfriend and Ian's friend, is sleeping in Louise's room, so Lena sucks and fucks him as well.
All through this, I have to endure the bullshit dialog of SIMP Ian being in perpetual guilt for fooling around on his fuck buddy. Ridiculous! I ended up holding down CTRL and speeding past most of the dialog lately as it is utterly ridiculous.
As for how well Holly "can take getting cheated on", that is irrelevant as Holly admitted to both Ian and Lena that SHE THOUGHT THE TWO OF THEM WERE INTERESTED IN EACH OTHER, yet she PLANNED to have sex with Ian anyhow... and why not, HOLLY KNEW IAN BEFORE EITHER OF THEM EVEN MET LENA and was interested in him first!
Again, the dialog is utterly ridiculous, even if our woke writer has plans for the future, the dialog is beyond annoying. I am on the verge of deleting this game and moving on as I am all about a good story, and this is turning into a really poorly written one.
At the very least, Ian should have raised the fact that Lena was sleeping with Robert, that is one he knew about. Perhaps bring up her hypocrisy in all of this and the fact that he knew Holly first, and him and Holly actually have the same interests and Holly hit on him first and he treated her well. He certainly shouldn't be apologizing to what is a FUCK BUDDY at best! Very annoying and insults the reader.
Hate sex is the best!
