This is exactly what happened in my relationship, too, and there was never any explanation for it other than that my libido began to eclipse his after about a month. After about one month of not being able to get enough of each other, it was like I had emptied his libido of all its contents, and his body takes a very long time to replenish the supply. That doesn't mean their desires for looking at other women diminish, but the inability to perform is not necessarily linked with their desire to look at other women. Some men who are completely impotent still want to look at and touch women. A girlfriend of mine dated someone like this.
The 'looking at pics' part isn't unusual at all. Men aren't like women. Heterosexual men always have interest in looking at other women. Always. Al-ways. ALLLLLways. AlWAYS.