I have been giving men blowjobs for 15 years now. I am not a porn star or anything, just been around the block a few times. And in my experience, I think it is their diet. A few of my ex's were beer drinkers, and they always tasted bitter. I just read that coffee, onions & garlic are a factor in bitterness too. My fiance isn't a beer drinker and I couldn't figure out why he tasted bitter alot, until now. He drinks coffee, and eats onions & garlic regularly, so I think we're going to change things to see what happens. Even with the whole bitterness thing, I still swallow because I know he gets really turned on by it, and it turns me on to know he's happy!