Karen's words do little to ease your guilt.

You stay silent for a while, digesting her words. She had a good point. Sarah was amazingly hot and all yours for the next few days, no strings attached. In spite of that, you’re beginning to feel like you’ve crossed a line you never truly wanted to. Karen’s sheer enjoyment of your coupling is the only thing stopping you from voicing your doubts. You wonder if you muster the same enthusiasm when it was Karen’s turn with Jakob.

“Alright babe,” you tell her. “I’m just scared of hurting you. I’m glad you’re enjoying this.”

“I’m loving it! Don’t worry about hurting me; you saw how much I loved you two fucking.”

“Oh yeah, you’re such a dirty little slut.” You squeeze her breast and whisper,” And I’m happy to be in love with you.”

Karen giggles in delight and turns her head, giving you a deep kiss. You return it willingly, burying your doubts deep inside. If Karen was enjoying this, you were damn well going to make an honest effort to do the same.