I just went back and watched the closing scene with Terry in this last episode...where the bust happens, and all of that. I saw such a parallel of his relationship with Zoe in his friendship with Terry. He obviously loves Terry, but can only handle him in small amounts. Zoe brings out that same fire...that he loves, but can't live with all the time...that's why he sent Terry away, and couldn't handle it if he looked back. That's why he misses the arguments with Zoe, but can't stay. Nobody could live with that tension/stress very long and remain sane and physically healthy, you know? But it adds a heck of a lot in small doses!
Enter his relationship with Gillian. While it has its moments of fire, the relationship is much less stressful with her. I think we begin to see a little more of why/what he considers 'pure' about her when we see the other friendships he has. His relationship with Gillian smolders...is always glowing and always gives off heat, sparks will fly once in while, but rarely does it turn into the full-on blaze like it does with Zoe and did with Terry.
It's fun to watch a campfire burn high and bright while gathering around it, but as the night wears down, it's the glowing embers that leave us feeling peaceful, and open to deeper things. If it turns into a forest fire, it gets ugly and frantic, and even the embers of a full-on forest fire can be disasterous.