I didn't like the Zoe scene for the same reason I didn't like the Tia scene. They are literally just sex. The way he behaves in those scenes is more like some asshole player who acts completely different than the person he is with Jamie and Shauna. He's going against Jamie's deal by being with them too, because he's not helping to make the girl happy, he's just fucking because he can.
It's getting quite clear which scenes are him cheating, and which scenes are him following Jamie's plan.
If you had sex with Shauna at the beginning
If you fucked Rachel in the office
If you accepted the pregnancy deal early in the game
If you fucked Liz in the basement
If you impregnated the first woman (and then did her again)
If you had the threesome with Rachel and the celeb
If you had sex with Tia or Zoe
You cheated on Jamie.