Keep my swimsuit on

I run into the water where Millie is waiting with my swim suit still on, attempting to bend forward to conceal that I have a boner.

"You are being a total prude." Millie yells and starts laughing as I swim up to her. I reach her, and I start threading water in front of her. I feel her hands tugging at my swim suit, and she laughs and says:

"Let's get these off."

I move my legs up so that Millie can pull my swimsuit off, and once she has gotten it off my legs, she holds it up over the water, dangling it back and forth in triumph, then splashes water in my face, and swims rapidly towards the beach laughing and shouting "keep away".

I start laughing too, and swim after her, and I manage to catch her ankle with my hand just as she reaches the beach. Still laughing, Millie makes it up out of the water, and we start rolling around, laughing and play wrestling for the underwear.

In our naked "struggle" for my swimsuit, we start to roll around, one on top, then the other, and eventually I end up on top of Millie, her legs wrapped around me, still play wrestling.

While my hard dick has been rubbing against Millie's body this whole time, I suddenly feel that it is pressing against something warm and moist. We keep play fighting, and then suddenly my dick slips into this warmness and moisture, and I feel it squeezing around the entire tip of my dick.

Millie suddenly stops laughing and moving around, her eyes open widely, and they look panicky. I then realise that my dick has slipped into my step-mom's pussy.

Millie starts tapping her hands on my shoulders, and in a panicky voice shouts:

"Matt?!!"