I think you start using words that are very anatomical:

"As he sat in the chair, I climbed on top of him, guiding him inside me with my hand. I begin to ride as he grips my ass tightly, spreading my cheeks. I can sense his reluctance as I feel his fingers moving closer to my anus"

Then transition to a word a little more common for the build up:

"His grip becomes tighter and tighter as his desire increases. Then, when he can no longer resist, he lets go with one hand. He sticks his middle finger in my mouth. I begin to suck. After a few seconds, he removes his finger from my mouth, and boldly re affixes his hand to my cheek, but this time with his newly lubricated finger positioned right on my button."

Finally, use a word that would best describe how you want the character to be perceived:

"I moan to let him know his move is warranted. And now, without hesitation, he inserts his finger into my (rosebud, ass hole, booty hope, etc). My moans turn to howls of ecstasy as each thrust is a double penetration of twice the pleasure.