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Step-Mommy and stepdad have been trying to get pregnant for a while now, but as step-mommy is a bit older than when she had you, they have been struggling. So they took all their savings and went to a doctor who gave step-mommy a special treatment with a high chance of success. The problem is stepdad has been called away for an emergency work trip, and step-Mommy is desperate to be pregnant and this is her last chance. She asks you to be a sperm donor. She will tell stepdad it's his, it will be your little secret, purely scientific. You agree, how could you say no to this? step-Mommy says you can just collect your sample and she'll sort it from there, or if you're really sure, the natural way has a higher success. You tell her will do it the old fashioned way. Secretly you're so turned on by the thought of step-mommy carrying your secret seed. Knowing that every time you look at stepdad, you'll know it's yours and not his. That power erases any discomfort, you WANT to impregnate step-mommy more than anything. She's shy at first and says let's do it for behind so we don't have to look at each other. At first it's awkward. You're both trying not to enjoy it, after a while you can't help thrusting deeper so you hit that special spot inside that makes her moan. She can't help it, she looks over her shoulder at you in shock, her moaning getting louder and louder before she begs you to cum in her and fill her up, and then she screams as she cums. She collapses in the sofa and is so shocked and sorry for how she behaved during, she just couldn't help it