Jemma has no poker face to speak of, and quite clearly she is torn between desire and disbelief.

"Please... don't tease." She understands academically that Pam thinks she tastes good, and has put together enough of the pieces to understand that in Pam's world, there's quite a lot of people who will happily give a vampire a meal in trade for sex. But so far as she can tell, Pam doesn't seem... attracted, much, to any of those people.