My understanding is it was a Gary Stu wish-fulfillment thing on the writers' part (they talked about it openly and were rabid fans of Kaylee) but it really wasn't fair to ask it of the actors because, yeah, there was laughably bad sexual/romantic chemistry there. I can't watch the scenes where they're literally face mashing because it gives me an embarrassment squick.

Jennifer had good chemistry with Sam Carter in Trio that was kind of adorable as well, but, well.