Watching porn does not mean has lost interest in you, not at all.
Some men just like it and that's ok, as long as it's not extreme or becomes an overriding habit.
I've watched it myself years ago but it was no turn on for me and frankly, it seemed ridiculous.
Top shelf mags and porn can be, I suppose, a release for men, mentally (and physically) and men can more easily separate loving sex within a relationship, from the act of sex itself, which many women can't do.
I'd leave him to it but please don't start thinking there is something unappealing about you, size-wise or any other way.
It might be an idea to ask him what he finds gratifying about it and talk openly about his thoughts on the matter, rather than him feeling it has to be a 'secret' to be indulged when you aren't watching or likely to be questioning him as to what he's watching.
Remember, it's normal...when not in the extreme.