I thought I would add my prespective as an anal virgin. I'm a chaser (or at least I will be when I take that first load). My point is that it is rude for someone to change their mind after telling the top that it is OK.
When I take my first poz cock, the second I bend over and he enters me, that is me saying I accept this cock in the spirit it is intended and agree to the consequencies. I dont think its right for me then to say no. In the strictest sense of the word I feel it is a breech of a social contract. In my view, if you dont want the load then dont take it raw, its that fucking simple!
If I protested I would expect the top to ignore and finish what he is doing. I expect I will get a gag put on me to stop that from happening but the fact remains, you do the crime, expect the time!
If you dont want the load then dont say your willing. Just be upfront from the start. If your not sure then tell the top that you are not sure and you will decide as it progresses. Then tell them ye or neh!. But if you say nothing then it should be infered that the top has a go to blow their nuts in your ass.