He seemed like a very functional porn addict to me. He was able to satisfy his porn need. I thought his pain seemed a little disproportionate to his problems with being a sex addict and felt there was something more going on. I thought perhaps his sex addiction was merely a consequence of his obvious intimacy problems. For some reason, he can't let other people in and it seemed to me that sex addiction had developed from that loneliness rather than the other way round. There's also a scene where his sister says they come from the same place, and she's suicidal, so that also led me thinking that both characters have some tragedy in their family background.

Yet the movie never really explores that part of their pain much. It focuses more on the sex addiction which felt superficial to me.

Or what do you think? Was sex addiction his biggest problem?

I agree the sex addiction must have been him acting out some pain that has lingered with him from earlier in his life. I think I prefer that the film kept me guessing about what had scarred him than to explicitly state it (the main reason I was interested throughout other than Fassbender's immense performance).

Also, I loved the relationship of him and his sister. Her being so flighty and him being so withdrawn. If you assume that they are somewhat similar to their parents, maybe the thing that left them hurting was something their mother did as a result of their father's coldness.

Well I thought of it in terms of his fear of incest. After his encounter with his sister in the shower, in the finds her red scarf and lightly and curiously pushes his bat through it. I saw this as either the fact that they've already committed incest or that it's on his mind--at least subconsciously--from minute one. Also, I think that the siblings were sexually abused as children, since the sister says "We're not bad people, we just come from a bad place." That would explain his sexual addiction, her suicide, and the tension at the notion of incest. McQueen selected these two actors because they were in a relationship in real life, that way, the actors would have a connection that hints to the audience, and the fact that these are sibling roles plays the incest card once more.

After thinking about it more, it seems obvious they both share a pain from their past, but they go about it in opposite ways. He has become cold towards people, anti social. Love and normal human contact has in many ways been replaced with easy sex. He copes with the pain by trying to block it out. That's why he doesn't want to have anything to do with his sister. She reminds him of the pain in their past life.

His sister deals with the pain in the opposite way. She feels it and generally tries to cope with it. She has become super-dependent on other people. You can tell by the way she tenaciously keeps calling him and how upset she is with the phone call from her boyfriend. She wants his love and support to help her through the pain and to keep her from committing suicide. He finds her dependence offensive and having her in his life reminds him of the pain he wants to forget.

That's the way I see it. I don't really think they've had an incestuous relationship myself. If I was to guess what the past tragedy was, my guess would be that they somehow lost their parents under tragic circumstances. They never mention any other family and they only seem to have eachother, even when she's in the hospital after trying to kill herself. If they had more close family, it makes sense they would've been notified too, but I got the feeling the director wanted to tell us they were all alone together.