My guess? Because sex is a more discrete boundary. Emotional fidelity is much harder to break out - what's the difference between close friends and something else all together.
I have a very close friend that's close enough to threaten other women when I was "pretending" to be mono (that's how I'm currently describing my pre-poly world). We have never had sex, never kissed, and never had a girlfriend/boyfriend type relationship. But, it is still hard not to describe our dynamic as less than romantic, deep loving connection. If you took a film of us at a restaurant, you would believe we were sleeping together. But, it's still hard to assess when a relationship passes from friendly to an emotional entanglement that violates the typical restraints in a mono or poly-fidelity arrangement.
Sex is a big bright line that is much easier to describe the boundaries.
My 2 cents.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”