Every leg both CA's and FO's sign a paper saying they are fit for duty. If you are not, or believe you can not complete the entire flight safely, don't do it, it actually is that simple. Some people just like to make it more complicated.

I would also likely have a difficult time completing the DEN-ONT-MCO. If I felt like I'd be fatigued, I'd call out, if I did not, I'd work it.

I don't think a big stink needs to be made, if enough pilots call out fatigued for it, it will change. Also it should go without saying, but FOs should not feel pressured to operate a flight if they feel fatigued just because the CA isn't going to call out.