Slime

Encyclopedia Entry:

A race of semiliquid beings that live mostly in plains and savannahs, usually in the vicinity of a town.  Their bodies are malleable and can assume a variety of forms.  They are intelligent enough to speak human languages haltingly, but their actions are ruled by instinct and they spend most of their time wandering around in search of human men.

When she finds a target, she will wait for him to get close before grabbing him.  Because her body is sticky, he will not be able to escape before she engulfs him from the waist down.  She will then use her entire body to pleasure him, and absorb the semen he ejaculates.