As an occasional browser of some of these sites, I think I'm pretty qualified to give you a really honest answer about what's going through his head.
He is probably thinking "crap...it's getting harder to get horny these days", so he is looking to see what gets him excited. My C would probably tell him to invite you to browse with him so that the sexual energy is shared between the two of you instead of just him. That takes guts on both sides but if you think about it, we are all getting to an age where things are weird. Our spouses aren't looking that great and our plumbing isn't working great either. I've know some older, overweight couples who use porn to get excited and literally use it as a "marital aid". Is this bad? According to Schnarch fantasy is not the path to enlightenment...so who knows? IMHO whatever keeps you together and happy is the right answer.
I guess my point is this...cut him some slack. Acknowledge that you know about it but immediately make him feel "ok" about it. Making him feel bad will cause him to take this activity "underground". When I've confessed some of my alternative interests to my W, she smiles and says "....like you are telling me something I don't know?...ha...it's ok and fine...you aren't a pervert". Then I feel much closer to her.
Now, I'm sorry but I haven't read anything except this post and I don't know your situation. Have you asked him what turns him on? I bet he thinks you can't rise to meet whatever weirdo (sarc) thoughts he has ( I'm saying this sarcastically because I've wrongfully assumed that my W wouldn't 'get' me if I told her my preferences).
Now there comes a point when his activity *is* bad for you too and the longer he does it underground, the more disconnected he's going to get.
Anywhere is walking distance if you have the time
-Steven Wright