Question

Seems like he couldn't get it up and felt humiliated.

i agree with saturn i just saw the film about a 20 minutes ago
he was probably ashamed(about getting it up) or mabey he was just so use to stalking women he never actually had sex with one

He certainly has some sort of sexual dysfunction. I'm sure that's to reinforce his sexual predator/serial killer psychology.

This film reminded me greatly of the 1965 British film, "The Collector" with Terence Stamp and Samantha Eggar. Stamp kidnaps his victim in that movie, but spends most of the film trying to convince his victim to fall in love with him with similar results. The very final scene of "Alone with Her" is practically lifted straight from the earlier film!

I really enjoyed this film, it was so creepy in parts I had to pause and go have a smoke!

In the Director's commentary he said people asked him whether Doug was unable to perform or ejaculated prematurely. he said he tried to make it obvious that Doug could not perform. he painted doug as a guy likely addicted to porn who could not relate sexually to a woman in a healthy way. this was possibly his first sexual experience, so his lack of experience, the fact that he was wound so tight, plus his dysfunctional view of women and sex, caused him to be impotent and embarassed.

i don't really know, it was like his dream to do it with her

"So, uh, you play the beautiful game... bros... brothers...brethren?"
-she's the man

cuz he was a virgin probably. heh

He is a voyeurist, he is getting sexually excited by act of watching. He thought that he can have a normal sex too, but apparently he couldn't. He is a psychopath, a serial killer. He cannot be normal in any way.

See Michael Powell's "Peeping Tom" from 1960.

Also, didn't he always wear a camera under his shirt to film her? I seriously doubt he thought that was going to be the big night. Where were all the wires?