You chose to join Emma on a Monday

"Monday," you say.

"Cool," Emma responds. "I think I can convince Jacob to be OK with sharing me."

This stings - because aren't you the one sharing your girlfriend with him already??! But you don't say this. You also don't tell Emma that the reason you chose Jacob is because you thought he was the guy who might be most likely to then share his lady with you in return. You might not like his girlfriend Isabella very much, but you can't deny that she's hot and way sluttier than Sophia, for instance.

You try to probe a bit more into Isabella and Jacob's sex life that night when you're fucking Emma. She's in a unique position to be Isabella's confident and Jacob's mistress, so she hears things from both of them. She tells you that they seem to have a healthy, happy relationship. She doesn't think her affair with Jacob has impacted their relationship at all - if anything it might be helping because she encourages him to show Isabella his desire and love for her in ways that Emma know Isabella will appreciate. Sadly you don't get any spicier details than this.