But it's not a male participant. It's a substance, not a being. And, again, it's balls-all easy just to set cum -yuri in the blacklist (or just cum on its own, if you're so [dis]inclined).
EDIT: And I wouldn't tag a few girls waiting for a reception from an off-screen sperm-donor to be yuri; only if there's no indication of active male involvement and a presence of active female engagement would it seem like yuri. Still not sure if I'd consider one girl spreading another toward the viewer to be yuri or not, though...
Updated