This may sounds kinky but my wife likes the idea. I suggested taking spermicide liquid lube and emptying half the bottle. The other half would be refilled by me, using a funnel to cum into the bottle when I jerkoff. The spermicide would kill any cuckold sperm as it mixed. When my wife masturbates to the thought of her bull, my cuckold seed would act as a lube to help her get off to her bull. My wife hasn't allowed me to cum in her in the last 2 years but she likes the idea of finding a use for my sperm even if its to lube her much bigger toys as she gets off to her Bull. Considering she gets so wet for get bull she doesn't need lube with her bull, there wouldn't be any risk of or sperm mixing when he fucks her.