They sound very confused about what poly is about. Perhaps they lean more toward the swinger side of the lifestyle and aren't the best couple to be testing the waters with?
If you feel uncomfortable you have every right to put the brakes on it. Talking is great but when a person or couple focuses on the sex more than your feelings it raises some red flags. imho.
Trust yourself and your instincts. You don't have to move faster than you want to.
Love laughs at locksmiths. ~ Proverb