Keep Finding My Sons Masturbating

I have two boys, Kevin 15 and Paul 14. The three of us live alone after their father walked out ten years ago. Nudity was never an issue in our house, when they were young and I used to bathe them we would all be naked. They always saw me naked, I thought nothing of it. Obviously this all changed before they hit puberty but they have caught me in just my underwear from time to time. The problem I have is that I keep discovering them both pleasuring themselves all the time. I'll either hear one of them in their bedroom at night breathing loudly while the bed springs are squeaking or I'll see one of them unawares. Neither one of them knows that I've observed any of this. I came home from work once and heard music coming from upstairs, when I called out to say hello there was no answer. I made my way upstairs and found that the music was coming from Kevin's room, his door was partially open and I saw lying on his back, completely naked, working a bottle of baby oil into his penis. His head was tilted back and he was breathing heavily. I must admit he did look very sexy and I found myself feeling rather flustered and excited. I watched for about three minutes before he exploded all over his chest. I backed away and left him to clean himself up. Last week I returned home early from shopping with a friend and found Paul home alone watching some internet porn on his computer with the headphones on his head. The volume must have been so loud because I could hear it from the doorway. Again he was totally naked and again it made me very excited. I could only see him from the back but I could tell he was having a lot of fun from the speed his arm was pounding away. Yesterday when I returned from work I looked out into the back yard and saw that our back gate was open. Great, I thought, somebody had probably broke through the gate and tried to break into the house. I made my way down the garden to check it out, and as I looked through the gate I saw Kevin out in between the trees and bushes behind our house, completely naked, kneeling on the ground wanking away. This got me really horny, I don't know why. Maybe it was the fact that he had no objections to being outside with nature, or the fact that he didn't care about being caught. I actually went into the house and pleasured myself thinking about it. Then this morning I went to Paul's bedroom door to wake him for school and when I opened the door I saw him face down on the bed fucking a stack of pillows. The ferocity and the aggression he was using to hump these pillows was unbelievable. He was like a pornstar or something, his little ass was clenched and he was moaning away. I don't know what to do, my once quiet home has now turned into a house of lust. If I keep catching them jacking off I might get carried away with myself and do something I regret. This is turning me on too much!