The only time the cum has a bad taste to me is with smokers -
Some of it may be the smell - the smoke makes their bodies and clothes all stink of cigarettes -
But on the occasions I have sucked off a smoker the cum tastes more bitter.
One guy that was a regular sex buddy growing up he started smoking and I did notice after awhile his cum tasted different - but of course I was so horny at that age it did not matter - I was so happy to be sucking his cock that if he spurted Tabasco sauce I still would have got on my knees for him!