The specific fic sounds like Entwined Branches by strive2bhappy on Ao3.. Summary: In a world where the government keeps a close eye on alphas, betas and omegas -- to the point of making certain everyone with these traits is mated by the time they turn eighteen -- it's no wonder the condition has become rather rare. It is, however, essentially unheard of to be unmated at thirty-three, but Jensen Ackles is an alpha who's good at staying under the radar. That is, until he's not and he's matched -- by bureaucratic decree -- to 18-year-old omega, Jared Padalecki. This is the story of how the two of them find far more than they ever thought they could.
There first night together includes the scene you described, with a very wet Jared grinding away on Jensen's lap, while Jensen lets him since he had initially decided not to consummate the marriage that night because Jared was so frightened.
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Summary: In a world where the government keeps a close eye on alphas, betas and omegas -- to the point of making certain everyone with these traits is mated by the time they turn eighteen -- it's no wonder the condition has become rather rare. It is, however, essentially unheard of to be unmated at thirty-three, but Jensen Ackles is an alpha who's good at staying under the radar. That is, until he's not and he's matched -- by bureaucratic decree -- to 18-year-old omega, Jared Padalecki. This is the story of how the two of them find far more than they ever thought they could.
There first night together includes the scene you described, with a very wet Jared grinding away on Jensen's lap, while Jensen lets him since he had initially decided not to consummate the marriage that night because Jared was so frightened.