"I'd love to, but..."

"I'd love to join in, but I'm exhausted. I think I'll just watch,"
You hear a muffled yell of surprise and confusion. Melissa scrambles out from under Becky, eyebrows raised. "Are you alright? Turning down sex?"
You smile contentedly, "I'm fine. And I'd love to join in, but I truly am tired. And right now, I'm satisfied,"

The two girls exchange wide-eyed glances. "That guy must have been one hell of a fuck!" Becky exclaims.
"You have no idea. I'll just lie here on the couch and watch. Don't mind me,"

A moment's hesitation before Melissa rolls Becky off her, onto her back. Positions swapped, the girls resume their fun, cumming onto each other's coiling tongues. You watch, feeling your arousal pulse. But you're simply content to watch.

Eventually you get up, bidding goodnight to your housemates. They ignore you, but you expected that. You go to your room, change into your skimpy little pajamas and wearily slide between your sheets. You are asleep in seconds.