Per Moonsong:
She goes on to say that arousal state is the key, but it seems pretty clear she's talking about a heightened state of pleasure and chemicals in the brain around that similar to orgasm. That's reinforced later when she says this about the pills:
So the pills work not just because they make someone aroused, but because they cause heightened pleasure and contain synthetic chemicals (and some ecstasy) that are the same as those produced during orgasm. That's also, by the way, why MC feels an orgasm-like pleasure near the end of Ch 8 when
he's unconscious. They are feeding him the pill so he can get Ruth's power, and the pleasure is from the pill itself.
It's worth noting that if
just being aroused was enough, MC would likely have Lucia's power on the manipulate path from what I've seen of that branch (though my memory is a little sketchy on the timing of some sequences). And I'm 99% sure that's not the case (the 1% is that he has it, but won't know it or be able to use it for a long time).
Also, MC doesn't need a fake orgasm detector, just the ability to make girls cum easily and reliably. And he definitely seems to have that (particularly now with Meri's power).