What could be causing it? Well, there are quite a few candidates: there’s evidence for correlations with obesity, diabetes, smoking, drinking, and lack of exercise, anxiety, and more; and the evidence for any of these is a lot stronger and a lot clearer than that for pornography consumption.
Based on my limited understanding of biology, CVD, downstream effects of insulin resistance, I would lean more towards environmental inputs rather than visual stimulation and psychosomatic effects. I significantly changed my environmental inputs 15 years ago and just speaking anecdotally, I've seen all the porn and do not have these described physical limitations.