That's exactly it- not only is he on top of you, pinning you but your ass is being rapidly stretched open each time he thrusts, and although your ass burns, he is pushing deeper and deeper inside. You weren't so sure about this when you felt his cock push against your asshole but you quickly realised you wanted more when you felt his cock move inside your body- and you heard yourself moaning in pleasure, just like he had after his first strokes. You feel his cock moving more freely inside your body as you relax and the sensation builds inside, until you feel like it can't get any more intense. A series of intense pulses from his cock, deep inside your ass, a hot slick wetness and you realise you are coming too, multiple shots across the duvet from your hard cock.
Afterward you feel strangely exhilarated, it just feels right despite the soreness between your cheeks. It is some time before you realise his cum is soaking into your underpants and tricking down your leg, but it is a thrill you will experience time and again as your curiosity continues to grow