It's a thick vibrator

Without warning, he pulls out a thick vibrating toy, shaped almost similarly to a butt plug, and forces it into your hole. It's thinner than your lover's cock, but thick enough that it nearly completely prevents his cum from escaping.

"Good, now we won't need to worry about any mess. Keep this in until we get home, we wouldn't want to be making any messes now," he commands, slapping your thigh with a laugh, "now get off my desk and go play on your computer, I've got some work to do."

It's now clear that he only has you here for the sex, so you spend the rest of the day playing online games while your father types away on his own computer.