^Khuutra
You can still see that in Gonarch, also known as the giant scrotum-crab thingie :) Mother (or father) of all those small headcrabs that like to violate the head of figures of male authority like scientists, soldiers and guards using what is clearly a "vagina dentata" :)
In HL2 it is even said explicitly that what they do is _mate_ with their victim.
Those guys at Valve are clever, and their writer Marc Laidlaw wrote some pretty disturbing horror novels (also rich of explicit and less explicit sexual undertones)