Not Quite Sure

About two years ago I happened to catch my son and daughter having sex. And I mean they were really having sex. Of course they were surprise and I was positively stunned, having no idea that they were doing this. I just stared with my mouth open, and then I excused myself.
About fifteen minutes later we all talked. They were both very contrite and I didn’t know what to say, except to tell them that I hadn’t had any idea that this was what they were doing. Although, thinking that I was being the wise mother, I had made the decision to put my daughter in the pill just about a year before as a precautionary measure, so in a way I blamed myself. They were quite honest and said that they had been doing it for a while and that they liked doing it together. I could see that it was very much a mutual arrangement, and all I could think of was that if I told them to stop, that they would probably just continue in secret and feel badly, which was not my intention to make them feel, as they were both being quite sincere. So I told them just not t let their father find out, knowing that he would never understand or be anywhere near accepting of this.
As I said, that was two years ago. This year they both headed off to college, the same college that is out of town, and they their father knows that they are roommates, which he thinks is only to save money. He is quite happy that the two of them get along so well. They have told me that they are living quite openly together and having a more serious relationship. As the keeper of the family secret, I am not quite sure just what arrangement they will be able to come up with in another few years.
Has anyone else had this experience with their own son and daughter?