I don’t think Sue Ellen once cheated on JR before Ray. I think it was strongly implied that she was VERY faithful to JR even though “he neglected her from the moment he brought her to Southfork .” As Miss Ellie put it.
That's the impression I got too - that Sue Ellen was faithful to JR until then - that's what gives her sleeping with Ray impact. If the time with Ray is the first time she ever cheated it's significant; there's an actual reason to have that happen. Based mostly on what was implied, and also to a minor extent, on the quality of the show back then, I think they did it for a better reason than "Let's make this scene so the the episode will last 48 minutes."
They did't just do it for the reason of showing Sue Elle cheating for the first time either. They did it for Ray's character too. Realize that of the show's main cast, Ray was the only one that the vast majority of the audience could identify with on a financial level or a social status level.
It was hard to identify with the wealthy Ewings and picture yourself living like them, but you could look at Ray and think: "Yeah, he lives about as well as I do." That was why Ray was such an integral character - the typical viewer could identify with him because he was one of them - not someone wealthy or famous, just a regular middle class man.
They did it to show Ray, despite being middle class and unpretentious, could still pick up women with the best of 'em. They did it to show Ray as being a stud.