Caught

I am a 43 year old female. I have been married for 17 years to a wonderful man. Ian, my husband, is very conservative. I was his second serious girlfriend and when we met he had only had sex with the girl he dated before me and she had been his high school sweetheart. I on the other hand was a little more adventurous when we met having had sex with multiple partners before Ian, though he thinks it is only two. I am a registered nurse and he works for a tech firm. We have two teenagers, Conor and Chelsea. They are actually the center of this confession.
Ian was away last weekend for a work conference and I had book club on Friday night. This left the kids alone which, given their ages, was no big deal. When I got to the cafe for book club two of the ladies texted that they had to cancel. As one other had canceled already, it left just me and Jackie. We decided to just have a glass of wine and call it a night. Usually, I am out until 11:00 for book club, but I headed home early and was walking in the door at 7:30. What I found when I entered my house was a shock to say the least. Music was playing in the family room which I suppose explains why they didn’t hear me for what I saw will forever be burned in my memory. Both of my children were naked and Conor was laying back on the couch while Chelsea was riding him for all it was worth. I couldn’t believe what I was seeing. After a few seconds, I flipped on the light and told them to both go to their rooms and get dressed. They were surprised to see me home so early to say the least. They were saying things like, “its not what it seems” and “mom you’re not supposed to be home so early” I told them I didn’t want excuses just go to their rooms. They were a mess of apologies and tears as they gathered up their clothes and ran to their rooms. I then went to our liquor cabinet, poured myself a large bourbon and sat thinking what to do. After I calmed down, I went to Chelsea’s room to be certain that her brother hadn’t forced her. By the look of what I saw I had no doubt that it was mutual but as she is younger I needed to be certain that she wasn’t manipulated. What I found out surprised me but in the end I was confident there had been no abuse. Since the event, I have spoken with them together and individually and got the full story as to how it all started. I promised them I would not tell Ian and I haven’t. There is no way he would understand. Now I sit and wonder how best to proceed.