My stepmother made me watch her have sex with her lover

How awful—how could this happen? I just happened to come home from school at the wrong time and heard moans and sighs coming from the bedroom. And my dad isn’t home—he’s away on a business trip. I open the door, and there’s a horrifying scene—actual adultery. My stepmom made me watch her have sex with her lover so I could appreciate her figure, and she promised me that I’d get to have my way with her later, too. So here I am, standing there, wondering why I’m even going through all this!