Can you elaborate on how it's even a 'sex scene'? Because honestly, it was like one kiss, followed by a fade to black; and a huge percentage of the scene was focused on the reliability of prophecy, the potential of stilling, the consequences of their plans, backgrounding the Eye, etc. And a sex scene, grounded in character work and plot, WHICH IT WAS, isn't inherently tawdry or gross. If you're suggesting it's because it was because it wasn't het, that's another matter altogether and one I won't engage with.
For what it's worth, explicitly showing them as having a romantic relationship with each other (complete with physical affection, teasing and so forth) means a lot to me as a fan of these books. I don't feel pandered to; I feel validated and affirmed as someone who didn't understand why younger me was sooooo drawn to New Spring and Siuan and Moiraine's relationship. It ramps UP the nature of their sacrifice, places it in even higher import to me.
I honestly just disagree that it somehow undermines the narrative and their character. The added fact that their relationship remained sexual, not platonic, isn't impactful on the crux of the matter - that if they're caught, if people become aware of their schemes (not that they had sex for the first time in years), they'll be stilled. That was the case in the book series and remains the same for TV.
The fact that you're talking about 'box checking' and 'shallow pandering' really does make it clear what the issue is, however, so I might leave it there.