Food, alcohol, medications, stress level, hormones, etc. Pretty much anything can have an effect on semen. That's why they tell guys to get a lot of pineapple juice in them if they're expecting a blow job within a couple days or so. Supposedly it makes the jizz taste sweet. I havn't tested that one out yet.
But I have learned that asparagus makes things HORRIBLE. I'd heard the rumor all the time, but now I know that it's truth.