I like it... I like it!! but I think I found a mistake:

Mark did hold on to her though, even as Charlotte became a madwoman, yelling insanely about her coming, her limbs flailing in ecstasy.

As she settled down, Mark put her down gently on the bed. His thrusting continued, each push driving her up onto the bed. “I’m gonna come too, Baby,” she told him.

She's had an O (as described by her insanely yelling about her coming and in the paragraph that leads to this O yelling description) So, shouldn't it read: "I'm gonna come too, Baby," HE told HER instead of "she told him?"

After all, she's had a great ride and I believe it is his turn at that point and the next paragraph is about him pulling out and getting ready to have his OOOOOO!

I also like how "cum" can be "come" in literary efforts like this. keep it going please!