I wasn't planning on making any kind of response, but because of all the comments I figured I would make a follow-up.
A couple of days afterwards when my son came home from school, he again just walked downstairs naked and started watching TV like there was nothing going on. This time I finally brought myself to ask him why all of a sudden was he walking around naked in front of me. He then told me during late summer vacation he was over with a friend and in his family they were very open with nudity. So much so that at one point one night every one of them had stripped off and they got him to strip as well. He told me he liked it and even though he walked around naked over there a few times, he finally got the courage to do it here. In a way I was kind of proud of him for being so comfortable with himself, but I asked him why he started masturbating so openly and he told me he figured it was no big deal. Everyone did it over there and that it was just a normal thing for them, and that he did it a few times over there as well.
Pretty much all I could do was shrug and tell him OK, if he wanted to walk around naked it was fine with me, but I would appreciate it if he tried to keep his masturbating to his room or the bathroom. He agreed and went back to watching his show, feet on the table and his knees spread apart. He stayed that way for a little while when he got up and went for the stairs, his penis sticking straight out. He was gone for a few minutes then when he came back it was hanging, but still kind of full looking so I knew he just made himself cum.
The next morning after I took my shower I went down for breakfast in just my bathrobe, feeling pretty uncomfortable because I hadn't masturbated in a couple of days. I finished eating and drinking my juice and I noticed it was a little late and he still hadn't come downstairs, so I went up and peeked in his room, and he was still asleep, on his back, morning erection poking up. Seeing him so open was all I could take and I had to make myself cum. I stormed back to my room and got back in bed, opened up my robe and started rubbing my wet pussy. It wasn't enough so I grabbed my small vibrator out of my night stand and started buzzing on my stiff clit. In no time I gave myself a thunderous, toe-curling orgasm that left me twisting around on the sheets. I was so horny I even licked it clean, which I never do.
I went back downstairs without tying my robe shut and stayed that way for a while until I heard him coming down, but then at the last second shut it and tied it tight. I thought about just saying to hell with it and going naked with him and be comfortable, but I really don't know if I could do it. There's no sexual feelings about him, it's just seeing him able to be so free about being naked and masturbating whenever that makes me want to do the same, but it's a pretty big hurdle to have to get over.