"Mother's Prized mare" has busty Christie Stevens as a seemingly horse-obsessed mama, finding a suitable gigolo (Stirling Cooper doing all-sex/zero-acting) on the internet to do studwork in fathering a child with her rather unappealing stepdaughter Nathalie Knight.
Dialogue is 100% idiotic, leading to an invisible creampie (viewers are expected to use their imagination). Of course, Christie cannot resist joining in for a threesome to add lesbian faux incest to the scene.
It was subsequently issued as a supporting segment on the DVD titled "Cock-Sl*t Conditioning" two years later. I guess the crude reeference to the daughter as a "mare" is considered okay by the pornographers.