Embarrassing question...

[naturegirlrocks.livejournal.com]

 So... I'm in the middle of writing my very first really seriously porny thing. (Yes, the former ones are child's play  compared to this I tell you!) But of course I'm stuck on the spelling on a word and can't continue. 

Ejaculation: is it "come" or "cum" (Brittish)? If both, which sounds best?