This isn't the first time I've compared particular art forms with something as primeval as sexual desires. Hell, last semester I had to give a presentation on The Passion of Perceiving by Christian Metz. It was essentially about how watching cinema is a primeval desire. Some examples listed were films like Psycho or Rear Window which is all about watching people, especially in a sexual way.
It appeals to the part of the brain that has been with us the longest: the reptilian system. It's always considered our deepest darkest desires because it's literally the deepest part of the brain. The arts, like photographs and film and everything in between seek to appeal to things that are human. Ecoporn is no different. Sexuality is a completely human thing to experience and when exposed with nature, it allows us to realize just how human the world, as a whole, is, not just the urban or rural areas of human habituation.
I know though, I'm going about this on a strange alternative level. The reading kind of argues a darker side of imagery. Supposedly they hide material circumstances of their creation by humans and whatever impact humans may have had on the landforms and animals they depict (p. 57). I can see it's use of rhetoric. After all, ecoporn is trying to make a statement. It's trying to get people to sympathize with it. I just think, why not? It's not really hurting anyone, and if they sell it, they sell it. I mean, I'd rather look at beautiful nature pics than aborted babies any day, and it's a more effective way than all that other stuff.