Hey there!
We've actually just added a lot of skills that will modify self/enemy arousal values, among other effects. The grapple bar is more fluidly used in non-sexual grappling stances, where each character is trying to gain an advantage in the grapple (Full Nelson and Ground Wrestle being two examples). In sexual stances, it's typically the case that one character can slip out of the "grapple" and the other character can release the grapple at any time.
The grapple status determines what abilities you have access to in the grapple stances, but doesn't (yet) in the sex stances.