Unintentional cuckoldry: I sneakily have sex with my stepmom's promiscuous lover while my stepbrother is away

Stepbrother's away, my stepmom's got a guy. I can't resist her curves and his size. In the bathroom, I'm sneaking around, giving her a deepthroat and some doggystyle. It's all about the thrill of being a secret cuckold.