What exactly was the problem with Gina's Marriage? *Spoilers*

To my thinking it was because he couldn't deal with her mastectomy and scars. When he pulled away from her and said he just couldn't I didn't take it to mean he was completely impotent; I took it to mean he just couldn't forget the scars (remember how she stayed covered up) and couldn't get it up for her.

I loved the scene just before Robert and Santiago comes face to face in the trucks. We see him breakdown and showing that he knowingly was partly responsible for Gina’s affair. In fact he might have known all along that she was having an affair and did not care as it was taking the responsibility of sexually satisfying Gina away from him. He was a trucker – he could get his kicks somewhere else and was OK with Gina having her lunchtime trysts. What he was not prepare for, was that it would kill his wife – or that it would be exposed to the world.

Ditto for the scene where they look each other in the face … and the acceptance that Santiago should take Mariana away.

No he was not impotent,her mastectomy just withered his sexual mood for her and of course that is the best and surest way to ruin a woman's self-esteem.
A woman like this would naturally feel like a "half woman" so this situation,along with him being frequently away from home,led his wife to find another man who wasn't turned on just by her physical looks.
Her husband was an irresponsible prick who kept accusing only Sandiago's father(and his kin) after their death,presumably because he thought he..."took advantage of her" due to "her condition",so in essence he kept regarding her as a cripple,both physically and mentally.
I guess he thought that this way he was exonerating his own as* from his guilt but in the end his conscience prevailed.

That was my main problem with the movie...there was little indication why her affair could be understood. Showing that he "couldn't do it" one time was too lame.

I'm not so sure it was entirely breast cancer and the mastectomy.

The father (Robert) was rough-hewn, but his greatest offense seemed to be his absence due to being a truck driver. However, in the scene where he roars off to "get" Santiago, he has an emotional breakdown, so he's somewhat emotionally cognizant.

I have to think it's something in Gina's unseen past. She was definitely a fish-out-of-water (Kim Basinger feeding chickens, priceless!), and she was lonely.

If we had a better picture of how she hooked up with Nick, then we would know more about her life, her marriage, and her character. This is a big gap in the movie as it seeds all of the other events.

Gina was lonely,
her life was boring, she didn't have much to do during the day
her kids weren't that nice with her
and her husband wouldn't make love to her
That was the problem!!

"Everyone is entitled to be stupid, but some abuse the privilege."