Riding a shemale cock!

You have a clear view of the couple in the room because the light is on. You see your mom sitting on top of a woman. Well you would call her that if you didn't see her cock sliding in and out of your mothers ass as she bounced on it. "Oh, god that's amazing!" she says as you switch between watching her tits and dick bounce up and down. The shemale moans, her hands on your mothers waist. You open the door a tiny bit more, trying to get a better view.
Before you realize it, your cock is in your hands. You try to stroke slowly, but utterly fail in the first ten seconds. Precum oozes out, allowing you to slide your hand up and down easier. You didn't know, at the moment, that the lovers you have been watching have stopped and only the soft shlick of your masturbating could be heard.
"Baby girl?" your mothers voice calls out. You know that they see you. Baby girl has been your pet name ever since you were young. You slowly open the door, cock still in hand, and are greeted by two pairs of eyes.
"Yes, Mom?" you peep out, embarrassed. She looks you over, cock still in her ass.
"What do you think your doing?" she says nicely, but it still scares the hell out of you.
"I was just-"
"Oh, I know what you were doing." she says, interrupting you. She doesn't say anything else. She just stares. After a few awkward seconds she blurts out, "Aren't you going to join us?"