You made great points, and I wanted to comment (agree) on all. I'll leave point #1 for last since it's the biggest can of worms XD
Two - Yes, he's got TONS of 'splainin to do. The way he treated her like she was an afterthought at a time when she was potentially about to lose a chunk of her hand (a musician, no less!) was inexcusable. Of course, Rashida wasn't exactly perfect herself, see point #3...
Three - You're absolutely right. In formal debate, her defense is known as "tu quoque" (Latin for "You too!") and is completely invalid. She responds to his accusations of infidelity by re-accusing him of going through her things, not once addressing the original accusation. This is like someone accusing me of stealing their bicycle, and instead of responding to the charge, I accuse them of cheating on their taxes. Granted, Theo was the biggest douchebag in the relationship, but I think she was responsible for a lot of their problems, and her behavior in that one case was probably meant to indicate that she added her own mess to the pot.
Four - I'm glad the film didn't end with a tearful kiss, reconciliation and wedding bells. Realistically they have so many issues to clear up, it's best left at an amicable meeting like we got in the end. It sounds like you're the professional so you would know if situations like this can be worked out. But I got the vibe that their relationship was so tentative, if it blew up before they even got married, it wouldn't stand a chance surviving til death do them part. I looked up the director Dana Adam Shapiro and learned that shortly after making this film he published a book called "You Can Be Right (Or You Can Be Married)" which, you might guess, is not exactly an encouraging handbook for wedded bliss. It's a series of interviews with divorced couples, focusing on problems that lead to breakups.
ONE - Here we go haha. Yup, I'm a man, and I'll admit that there is an immediate obsession factor with mysterious women doing nasty things. I think this is just because of how unusual it is in society. Tons of male pervs in the park having fun under their trenchcoats, but since when has there ever been a pretty blonde in a cute tennis outfit doing the same thing? So it's not necessarily a hormonal obsession (although that does figure in) as much as it is curiosity over the unusual.
With that said, however, I don't think most men would take it to the level of obsession that he did, unless of course they were in a very unsatisfying relationship as he was, compelling him to live vicariously through the racy sexual exploits of others.
BUT here's where I'm going to throw a curve ball at you. I don't think his obsession was really sexual. I think he became obsessed with the notion of infidelity. Notice how he became very interested in her wedding band, the man's, and the man's identity. Theo wasn't just some voyeur perv zooming in on her goodies. Rather, as Theo's own fears over infidelity became more pronounced, he began to stalk this woman with the intent of studying her extramarital affairs.
You can see how disgusted he is with his married friends at the bar when they talk about other women, or about their lackluster lovelife with their wives. Theo desperately wants to believe that marriage is the magical "monogamous" answer to loneliness, and when he finds Subgirl, he begins to swallow the bitter idea that fidelity is a myth. Like looking at a gory car wreck, he can't pull away from the thought.
...until the surprise ending which snaps him out of it. I agree it was a "happy" ending, or at least as happy as one could be within this context.
Reading through the comments I am relieved to see someone else who interpreted the film the way I did.
I felt it was a very real portrayal by both Chris and Rashida of a couple struggling with coming to terms with what a long term commitment really meant. It wasn't just a man neglecting his girlfriend to chase someone else, as both characters clearly had flaws and justified reactions to their situation. Theo just took it too far.
BUT here's where I'm going to throw a curve ball at you. I don't think his obsession was really sexual. I think he became obsessed with the notion of infidelity. Notice how he became very interested in her wedding band, the man's, and the man's identity. Theo wasn't just some voyeur perv zooming in on her goodies. Rather, as Theo's own fears over infidelity became more pronounced, he began to stalk this woman with the intent of studying her extramarital affairs.
What you say here is, for me, what the film was about. You could see the doubt and uncertainty in Theo even before 'subgirl' became involved, he questioned the idea of marriage and whether or not Nat was still attracted to him constantly.'Subgirl' was part of that. I am not saying that he wasn't excited or attracted to her or the mystery around her, but I do think that his obsession with her was rooted in his own uncertainty about monogamy and the fact that she appeared to be having an affair, hence the title.
The obsession was not just a choice. It's a sign of a psychological problem. He has issues that should be investigated and resolved. Otherwise he's going to keep having problems.