thank you VopT for linking to the wiktionary thesaurus for semen because there are some really funny slang terms for cum that i've never heard before
slang for semen
•cock snot
•fuckjuice
•jissom
•knuckle children
•mad milk
•scum (chiefly US) (as an american i've never heard anyone call their sperm "scum")
•white pee (childish)
bonus slang for pre-cum:
•Cowper's fluid