Women are emotionally driven. If you can't engage their emotions "halfhearted" is all you will get no matter how much RP or PUA tactics you use. Most RP and PUA advice that I have been seeing for the past year or two is all creating transactional situations. If I do 1, 2, & 3 then she will respond with X, Y, & Z. But if she responds with A, B, or C, then I will "next" her with 4, 5, or 6. Yet the same RP community will state that "You can't negotiate genuine desire",...but somehow they don't make the connection between the two.
A guy needs to be able to read her and know how to spot the indicators of genuine desire,...and if it isn't there, then don't bother. Even if you manage to coerce sex out of her at some point it still isn't going anywhere.