It's not much about the time, more about the interactions. It would obviously be more believable that Joyce would be more comfortable after you keep showering her with compliments, and giving her the attention your father hasn't given her for years, adding the yoga and massages to that.
For Isabel, let's just say you're there for her. Obviously, helping a girl during a breakup doesn't mean free sex, but this is a porn game so. Being there for a girl, listening to her and just ... respecting her and treating her well is bound to make her more comfortable with you.
For Ruby ... I honestly don't know. From the interactions all I'm getting is that she's thirsty as fuck and probably a nympho, doesn't necessarily mean it's badly written, just gotta read through the lines sometimes. Her whole situation, bedroom state and overall attitude makes her out to be the exact opposite of what you'd call an innocent girl, not surprised she would react that to way to skin on skin contact and spanking, plus the whole "caring for her" is starting to show its effects on her, adding to her old relationship with Connor.
The thing here isn't that you're just spending more time, you're spending more QUALITY time with them.
You never happened to be sexually attracted to someone even though you weren't from the start ?