More common than you think.
I have been in a sexual relationship with my mother going on 15 years now. It began when I was 28 and she was 49. My step father was dying of cancer at the time it began, and although there was some guilt about what we were doing initially, it turned out to be the best thing that had ever happened to us.
After my stepfather passed away, my mother asked me to move upstairs with her, and from that night on we shared the same bed frequently for nearly five years. My mother is still the best lover I've ever had (even better than my wife), and she claims as much as well.
No one in my family knows, but my youngest sister is suspicious, being she has nearly caught my mother and I a few times. We were in a committed relationship for about 5 years before we both decided that if we wanted to continue being intimate with each other we had to live apart for appearances sake.
We don't make love as often as we did, but we still make time to meet in private. My mother never fails to tell me how I will always be the first man in her life, and she still doesn't clean up when I finish inside her, being she knows that is what I like.