A club huh?

She didn't get a chance to cum, herself.

Lose the comma (maybe), or just lose the comma and "herself". It's a bit repetitive.

She didn't give him a disapproving glare. But neither was it the dull grin of post-coital bliss. Her expression was neutral.

You can combine these three into a single sentence if you'd like. It might just make it flow better. No matter what though, you should combine the first two sentences for better grammar:
"She didn't give him a disapproving glare, but neither was it the dull grin of post-coital bliss; her expression was neutral."

"I know how you feel. I had to carry twins," Cake said.
"Oh, right. I can't even imagine what that must have been like," Cadance said. Cup Cake just shrugged.
"It won't always be easy. But I can promise you one thing," Cup Cake said, "You'll never regret it."
...
"So... did you... bring it?" Mrs. Cake asked

You go from calling her "Cake" like how Harry and Draco call each other "Potter" and "Malfoy", to her full name (Cup Cake), and then over to Mrs. Cake. I'd suggest choosing one and sticking with it.

This is getting good quickly. I like it!