Your semen.

"What's up?" Selas asks, walking down from the tree she was perched upon. "That was fun to watch, by the way. Are you going to do that to her again later?"

"If we're going to do this, we need to talk about my balls," you say flatly, ignoring her question. Selas raises an eyebrow at you and smiles. "You said that you made them pump enough cum to fill up a woman. And if the hospital was any indication, you are going to fuck with every condom I wear. So, and I can't believe I'm saying this, I need you to make me sterile."

Selas wraps herself around you and caresses your erection over your trousers. "Aw...but where's the fun if there's no danger? The risk just makes it so much more exciting."

You refuse to be swayed, as your brain presents this as the most logical solution, given your lack of choice in the matter. "You'll still have your fun, because only the two of us will know about it. The women you make me sleep with won't, and you'll get your kicks from their panic."

She ponders your request for a minute, tapping her finger against her lip. "Alright, how bout this?" she says, extending her hand out to you. "I turn your cum into demon cum. You won't get any humans pregnant, but you'll probably, definitely, get me pregnant. And I get to call some girlfriends over to fuck you. They've been on a mommy kink for the last couple of decades and they'll owe me big time if I can get them knocked up."