Caught Masturbating

I am Dan 32 divorced and now living with my Mothers Karen 63 also divorced. Mom is 5'11" tall 105 lbs medium size breasts. She a lawyer by profession, She is a partner in a large firm. I am by trade a denist and have my own small practice. My ex wife ran off with her boss and as we had no children, it was a simple divorce she got half of everything. I sold the house to pay her off, so I could keep my practice going. So I have no more to pay her, we had lump sum settlement. Anyway to the story, I was home on a Tuesday evening laying on my bed in my room. When I heard a moaning coming thru the wall from my Mom's room. I was going to investigate, when I realized it was not a painful moan but a pleasurable type of moan. I went to her door and open it just enough to see in. There I saw my Mom masturbating on her bed, I watch her for a while. Then I returned to my room to take care of my now hard cock. I was well into it, when my Mom open my door to tell me our dinner was ordered and would be here in 10 minutes. She caught me in mid stroke and just said finish up and washup for dinner.
I was shocked at first and then so what it only natural. At the dinner table I said Sorry Mom for shocking you. She was I wasn't shocked, I do it myself and I enjoy it. To which I replied I know I heard you thru the wall. "You did" she replied "Why didn't come in and we could have gotten each other off". I said "I didn't think you would want your son to do that". That absurd was the reply, "It's only sex between 2 consenting adults and I'm far to old to get pregnant and it would be our secret". So I replied how about after dinner we give it a go. That how I started sleeping with my Mother.