Please use lower CFG scale. Lower CFG(like 4-5) results in more cum, maybe. ^_^ Sorry, it is not stable enough. Due to my lack of ability, I tried for a while but did not achieve good results in this concept. Maybe you could try it with some artist tags. You can add "projectile cum" in negative prompt to avoid cum in background somehow (not always). Warning: Sometimes generate really bad anatomy pictures
Blue color -> More cum (I don't know why)
Horizontal image -> More cum (I don't know why, either)