I think for this to work, honesty really is the best policy! I've been in a similar situation, and lucky for me, I was able to answer the question honestly!
"Baby, sex with you is on a whole nother level!"
I really hate lying, so if we ended up being with someone that I did enjoy more than her, that would be a problem! But I really don't ever see that happening because we both understand that emotionally we love each other more than anything, so that puts a passion into our sex life that can't be matched by random swinging!
As far as sex goes, we both think it would be ridiculous to only have sex with 1 person for the rest of our lives. I mean, as great as the sex is, that would still suck! I love a good Rib Eye, but if you told me that's all I could eat for the rest of my life, it would get old really quick!
So keep your entree hot at all times, but pairing them with lots of yummy sides!