At first, it was a little confusing. Who is this random couple enjoying loud sex at work? The author could have taken a Sex in the City route and just let this be a side character drama (ala Samantha) to work in hentai even though MC always gets blocked. Instead, ch 4 shows coworker guy is something of a predator who only likes women who are already attached. My larger suspicion is GF might be putting out because she wants coworker guy - he won't come onto her unless she seems to be in a serious relationship, and he just got the sign that she's his kind of girl (and she looked conflicted about him seeing her boyfriend). Maybe she wanted to try her BF first because she felt guilty about getting hot for coworker.
Either way, "A Knowing Sister" seems to be the experienced younger woman. She's more knowledgeable about sex, came prepared with condoms, offered them when he was in a pinch, has cared about him for a long time, and was supernaturally accepting about the drama in his apartment. I suspect, we're getting set up to think she's a better match for him. But if this manhua has legs, there will probably be another guy on her side too. She did use at least one of those condoms. It's unclear that she feels attached to that ex in any way, but these multiparty affairs tend to play out either as tease or cheat plots with plenty of conflicted emotions.