Ain't no way that's his child. The wh*re left it there and went away for 5 years possibly f*cking god knows how many other dudes. In the end lil simp there raised a child not his for 5 years in the same old apartment in hopes she would return, claiming he would forgive her for everything wtf lol
Sounds like a lot of speculation and bias on your part. Unless you know Vietnamese, we can't really confirm this.
My theory is that she got pregnant with the MC's child through the artificial insemination and then ghosted him until she got a round belly. She had a fake boyfriend to fool her mother about the father (well, fake as in they probably didn't do anything since she kissed him once and was super disgusted by it).
By the time she meets MC with pregnant belly, cue all that drama between the mother, aunt, and so forth. I think the author put some drama on the fake boyfriend by making it seem that the FMC never told the MC about her pregnancy when she ghosted him again. But I'm pretty sure MC is the father and fake boyfriend being the father is a red herring.
So after all the drama, they have pregnant intercourse (wow first time I've seen in manga, what a rare find), she gives birth, and then she disappears yet again leaving MC with their child. Being as dysfunctional as she is, she probably couldn't handle being a mother and needed time to sort herself into being at least a sane and normal person, but she trusts MC to wait for her, as he did all those years ago.
And no, I don't think she went screwing around with dudes. Especially given her behavior around other men as being repulsed and disgusted by them, all except MC.
I think this was the story the author was going for. Of course, this is all what I could pick up from skimming the Vietnamese chapters and reading the first 16 or so English chapters
Edit: I've just googled picture translated the last 3 or 4 chapters and the chapters in the early 30s (when they separated), and I am 90% sure FMC has MC's baby. FMC and MC had a falling out, and once she got pregnant, she ghosted him. MC wanted to be a father and a husband, but FMC just wanted to use the baby as revenge against her mother and won't let anyone interfere with her or her baby's life.
As for the supposed fake boyfriend, it was an arranged marriage by the mother (mother has clout to get her daughter married), but FMC has not done anything with him nor does she plan to go through with the marriage. She's keeping both men out of her life despite their feelings (both guys like her).
As for the end, she leaves MC and her baby since she's dysfunctional. She stabbed her mother and used the baby as revenge against her; not exactly mother material, and she knows it. With both her mother and the baby gone, she is free from any control and obligation (she knows MC will take care of the baby), and so she disappears again. She disappears to start a new life that she can choose freely. Cue the five year skip, and she returns to the old apartment, where MC and child wait for her. MC will accept her again, letting the past 5 years be water under the bridge.
As for what she did during those five years, we don't know. But given her past behavior and actions, we can probably assume the following: she tries to fix herself from being dysfunctional, a bit of soul searching and what she wants to do in life, and then realizes she misses MC and her baby and goes back to them to start a family. And no, I don't think she dated (let alone touched) anyone since she truly loves the MC and has like a phobia to all men that approach her (except MC).