I think Eric thought he could keep it hidden from her. When she found out, Eric lashed out by causing psychological injury to her; the "sassy piece of jailbait" comment, removing Francis from her realm(the club), interrupting her when she's with clinics. Eric mentions he has to keep his feelings in check, "that is what the contract is for", but he obviously has feelings for Zoe that he's dealing with.
Also, there is a gap of time unknown to us between the field scenes and the club scenes, and there had to be some disconnect that caused Eric to stray like he did, or to even contemplate an "affair" with Zoe, prior to getting involved with her. What that catalyst was is not known.
Also, I think that Christna and Zoe were lovers;re that passionate kissing scene. There could be jealously going around for everyone involved.
Besides the alternate answer I gave about the platonic relationship teetering on something more, I don't think there is an entirely right or definitve answer for his motives.
I hope this makes sense, it's almost 3AM now.
There is no night as deep as this
Inevitable mind's abyss
Where I now dwell with foes alone
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