Spoiler (click to show/hide):
Sure, makes sense for me. Roll: Natural 20!
Your son invades your body, and you feel him start squeezing into your womb, squirting a slime internal condom as the Nightmare begins to mount you. The monster spreads you wide, and you almost scream in ecstasy or pain, you can't decide as you're stuffed to the brim. You're a quivering wreck before the nightmare is even close to being done, your son squirming around on your chest and sucking on your titties. A few tentacles reach down to stroke the bulging womb standing out of your stomach as the nightmare stretches you out.
The horse thrusts against you, and you're smacked against the ground by the force of the thrusting, and you're reduced to orgasmic bliss for several moments as the horse has his way with you. By the time you feel the horse quickening his pace, you let out a loud grunt as boiling hot cum tries to penetrate your protective goo. You begin to inflate, the thick seed unable to penetrate the goo your son applied to your cervix, but the force is enough to force it into your womb, like a condom.