I was 13 at the time
To make it short I was going through puberty at the time but my older cousin (a male) just turned 15. I was staying at his house and we were both to sleep in the same bed. He was in his boxer underwear and I had tight underwear on too. Our of the blue he pulled his semi erect penis out through the cut opening and came right out and told me I had a cute tight butt and wanted to "fuck it".I did not know what that word meant then. He explained it all and for some odd reason the thought just turned me on so I told him okay. He got some tube calling it lube. I pulled my underwear off getting onto his bed face down while that slippery stuff he rubbed on his now fully erect penis. It was about average sized say five and a half inch long. He came into the bed and positioned himself over me telling me to relax. I did so and began feeling the head of his penis slowly inserting into my rectum. Once fully in he bends over kissing my neck. The whole time he was "fucking" me I can feel his hot breath in the back of my neck and sometimes ear. I to this day remember what it felt like going in and out of me thrusting away and moaning quietly near my ear. It lasted for about two minutes and then felt him ejaculating inside of my rectum.
That probably happens in a lot of households.