I was 17 when I walked into the bathroom and caught my 15 year old brother masturbating. He had forgotten to lock the door and caught him sitting on the toilette with his pants and underwear down. I closed the door and saw his full erection as he stood up to pull his pants back up. I was so sexually aroused I never made any attempt to leave. He looked so embarrassed I knew he would never say anything about the incident. I also knew he was scared stiff about me telling my Mom, making me promise to not tell her. Both from a strict Catholic family, things like that were considered sins let alone the embarrassment of being forced to publicly confess it. That's how I ended up convincing him to have oral sex behind our parents backs. First time was embarrassing for both of us, but soon we both began enjoying it. We stopped doing it after a year and know that we are both married, I know we both have lustful thoughts about each other.