Instincts are a powerful thing. Deep down, every woman wants to submit to a dominant man and have his babies. That said, they don't just want any asshole giving them commands. They want one they love and trust.
Who does a mother love and trust more than her own son? What could amplify the pleasure they both experience like the social taboo and profound intimacy of incest?
Your mother is just like any other woman. She was born to serve, to serve YOU. Once that line has been crossed and you've finally had sex, the best thing you can do is firmly establish your new roles. Being her son's sex toy will kick you mom's libido and ovaries into overdrive. Next step is to stake your claim by putting a baby in her waiting womb. It's what she was made for, after all.