They want him to breed the woman in front of the man

"You're going to find this odd." The husband said. "But knowing how your race is so sexually gifted and can breed like rabbits with anyone we were hoping to help play out a little fantasy of ours."

Kazex raised an eyebrow. "Huh?"

The wife smiled sinisterly. "We want you to knock me up when my husband watches."

Kazex was taken off guard by that.

"Let me explain." The husband said. "We always wanted to do one of those cuckold things. But even more so we'd like it if the man who fucks my wife gets her pregnant."

The wife continued "But I would not like to give birth to any more human babies 'cause it was painful. I've already birthed two times and don't want to go through that again."

"But then we heard about how you goblins can breed with any humanoid race, how the babies grow fast inside the mothers, and how nearly painless the birthing of goblins is."

"So what we are purposing is you knock me up while my husband watches and maybe even both of you can fuck me while I'm pregnant. Afterward, when I give birth, we can let you go and keep the babies."

"We might even give you a bonus. What do you say?"

Kazex was surprised that they would make an offer like this. He felt they must have been big perverts. But getting a chance to breed with a hot wife and getting the goblin babies he wanted to start a tribe is an offer he could not refuse. "Alright, I'll do it. But you better keep your word that I leave here with the children."

"Of course." The husband said. "We will also let you choose how to fuck my wife."