[Rationally, he knows she has a point. He's never one to admit he's wrong though, so he turns away, fighting the urge to throw something. He maintains his distance a bit, listening to her until the end and slowly starting to feel less certain of why he even bothered coming to her to begin with.]
[But it's her final inquiry that makes him look to her again, this time confused less with her assumption, but more what his immediate answer to it is.]
... I am... envious. But not for reasons you presume. [Collects himself and responds less hesitantly.] I don't believe that one can truly be satisfied with a single person for all their lives. The notion that marriage or commitment is even healthy is laughable, a best. Damaging, at worst. Who you share your bed with, or don't, it matters little to me. It's the why, that makes me wonder.
[Shrugs.] Call it a sort of neurosis, if you wish, but you're a riddle wrapped in an enigma that I had sex with. You'll forgive me if I'm prone to suspicion.
No the envy... That is something I dare say you truly wouldn't understand were I to elaborate. [He doesn't get it himself, fully. But it's not that she's with someone else that bothers him. Its that that person seemingly knows her... accepts her for who she is. And he didn't leave. But she'll see the hurt in his eyes very briefly before her clicks his tongue and stands near one of the walls, hands clasped at his sides and shaking.]