My son had recently turned eighteen when I started noticing him doing things like peeking in on me while I was taking a bath, watching me get dressed in the mornings, checking out my butt when I'd bend over, and once I even found a pair of my used panties under his bed.
Then I noticed in the evening when I'd wash the dishes, he would often come and rub my shoulders, but he'd slyly try to lift the front of my shirt enough that he could look down and see my nipples in the process. He was as tall, if not a little bit taller than me, so I'd feel him breathing down my neck as he was trying to get a look. I even mentioned it to him once, saying "oh your breath on my neck tickles" and he backed off, I think embarrassed.
I have always had large breasts, and rarely wear a bra under my shirt at home, so I'd frequently notice him looking at my nipples through my shirt.
One morning he called me over to the linen closet because he was going to shower and needed a bar of soap but couldn't find it. I was still in my nightie, which only covered to about mid thigh, and I bent down to find to soap in the closet and I turned and caught him bent over behind me, looking up my nightie with his face practically in my panties. It startled me, so I stood up quickly and he did too, trying to pretend he wasn't just looking up my nightie.
Then one day I heard him in the bathroom shouting expletives, so I knocked on the door and asked if he was ok. He said he had cut himself by accident while trying to shave. I expected he had cut his ear or lip, so I opened the door and quickly realized it wasn't his face he was shaving.
He was holding a towel on his crotch, trying to hold pressure. He was obviously embarrassed, so I just matter of factly tried to put him at ease and I asked if I could see. It turned out he had just nicked the skin at his groin, but it was then I actually caught myself getting aroused when I saw his smooth, decent sized penis hanging in front of me.
It had been years since I had seen it, so I wasn't expecting the little thrill I was feeling. I had to avert my eyes and just told him to hold pressure on it for a couple minutes and it would be fine.
I left the bathroom and started thinking we should have a talk about puberty, sex and those sorts of things since my little man was starting to shave down there.
So I sat him down and we talked that night and of course he acted like he knew everything and didn't want to talk about it, but I felt like it was important to keep that door open. Then I jokingly told him if he needed help shaving in the future he could just ask me, which at that point we both laughed, but I felt awkward after I realized the joke I had just made with my son.
He was just growing up so much, I didn't see him as a kid any longer. I guess I felt more comfortable talking like that around him.