I dont think having that pleasure damages anything. As long as they respect each other. If they like each other, respect each other, there is no manipulation during the sex or lies or carrot dangles as far as relationships, they will be JUST FINE having sex. Your having sex with someone you like and who likes you and respects you, and you happen to be physically attracted to one another. What's wrong with that? No one is taking advantage of the other or treating folks like chit.
That's the exact mindset that would lead two friends to having sex, sure, and it seems all dandy in theory. In practicality, however, once the deed is done and emotions & expectations enter the equation, things tend to play out differently.
Remember that OP said this chick was into him and he rejected her. Sleeping with her can rekindle those initial feelings the chick had and she could be fine for a little while after sleep together, but women will be women and you'll never know for certain what will happen to the dynamic. Which is why I asked the OP what he values more, the friendship, or a potential ONS/FWB.
You provided a good rationale. But you're also a male and so am I. It's easier for us to have no strings attached sex with a girl we're not that into. Gender aside, if the shoe were on the other foot and OP was super into this chick at one point and
she friendzoned him, but down the road gave him an opportunity to start sleeping with her and his options were limited at that point, I'd say there's a good chance he gets attached
and the friendship dynamic changes once he's fulfilled her need for him.