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He may be looking at the sites in questions as a way to distract himself from the fact that he is having the low libido.
Also, men are stimulated easily visually. He may be drawn to this sites because they offer a sense of stimulation w/o any strings attached (like going to a strip club)
The behavior can also become addictive in a sense that if it offers him a safer channel to deal with his issue. It may be a defense mechanism of sorts (where performance anxiety is absent)
Some other factors that can lead to low libido are decrease in testosterone (a blood panel test can show that), depression, certain medications, prostate problems, etc.