Caught My Sons Fucking

I normally wouldn't peek into their room but it was late and I was walking to the bathroom and heard weird sounds. I've overheard the likely sounds of jerking off before and done nothing about it - this was more. Like, headboard hitting the wall sounds. So I looked, and i couldn't fucking believe what I saw. My older boy was on top of the younger, pounding away as hard as I've ever seen any two people have sex in my life. I didn't do anything about it, partially out of shock... partially out of arousal. I admit I wasn't and still am not sure whether or not it was even consensual. I feel like a horrible father, but I want to catch them at it again so badly.