I quickly spread the lube on my dick and over her sphincter. As I did, I took the opportunity to slip a finger in to see if she was really ready for this type of intrusion. Her sphincter muscle relaxed as my finger slipped in, so I thought she was ready. I took my position between her (and Jason's!) legs and lined up my cock with her pucker. I pressed the tip against her ass and she said, "Go ahead, but take it slow." I began to gently push into her. At first she tensed up, making it difficult, but she slowly relaxed and I began to work my cock into her ass.
She moaned and stiffened as I slowly pushed into her. Once I was fully in, she said, "OK, hold it there for a minute." I held still while she got used to the feeling. After a short time, she said, "OK, that feels good." She began to move on Jason's cock as I began to thrust slowly in and out of her ass. It took us a few moments to find a rhythm, but once we did, the sensations began to build. Jenny began to moan almost immediately, babbling incoherently. "Oh, god, I'm so full! So good...ugh...oh my god, keep fucking me!" and on like that. For my part, I had never seen such an erotic situation. I could hear Jason moaning, and I was incredibly excited, both by the physical sensations of the three-way and the almost constant moaning and thrashing of Jenny. It became difficult to keep pumping into her tight ass, but I managed. The feeling was incredible. It was hot and tight, and her muscles were almost in constant spasm around my dick. I didn't think I could last much longer, despite have cum just a few minutes earlier. Fortunately, I could tell that Jenny was really close, and Jason didn't appear to be too far behind.
Suddenly, Jenny's vocalizations went from a moaning to almost a scream, a single syllable held and keened out while her body went rigid and her muscles went into spasm, contracting randomly around my cock. Jenny calmed down for a second, then went into a series of spasms, having orgasm after orgasm, until she finally collapsed against Jason, completely limp. At about the same time, I could feel Jason begin to fill her pussy with his spunk, and that sent me over the top, causing me to dump my load into her ass.
As my breathing calmed down, my cock began to slip out of her well-lubricated ass. I looked around saw the towel that Jenny had brought from the bathroom and slid it under us to catch the outflow of juices from Jenny's well-used pelvis. Jenny, recovering slightly from her climaxes, began to growl deep in her throat with the aftershocks of her pleasure. Her eyes were still closed, and it appeared that she was slipping slowly into sleep. I used the towel to clean her up and then myself, and offered the towel to Jason. He cleaned off his shrinking but still-impressive member, and we carried Jenny into the bedroom and tucked her into bed. I gave her a final kiss as she drifted off into sleep, a smile playing across her exhausted lips.