He isn't turned on by his girlfriend

"Doc, my girlfriend isn't doing it for me anymore, and I don't know what happened!"

"Calm down. Just lie on the sofa, and tell me what exactly is going on, Mr...."

"Just Nathan. I'd like to keep this kind of anonymous." He got onto the couch and lay down, fidgeting slightly.

She rolled her eyes slightly, but said "Of course. Now, how about you start from the beginning."

"Right, right. So, my girlfriend and I got together a month ago, and the sex was great, right? But recently I've stopped being interested, I've even had some trouble getting it up, and I never have trouble with that! I get hard just from seeing sexy women on the street!"

"I see. Tell me, how well did you know her before you started going out with her?"

"We were really good friends. I was friends with her boyfriend at the time too, although, heh, not so much now."

"Did they break up before or after you got involved?"

"Honestly, I kind of started flirting with her while they were still together. Bad manners, I know. Let's just say that they broke up and we got together in the same day."

"Did you have sex with her that day?"

"Hell yeah, it was awesome too. But what does this have to do with my problem?"