Xeno - Something Alien
This is Bestiality, dicking down a Xenomorph would be Xenophilia.
Unless we're talking about tag usage on this site (which I doubt), you're wrong.
From dictionary.com:
xeno-
a combining form meaning “alien,” “strange,” “guest,” used in the formation of compound words
ORIGIN OF XENO-
Combining form of Greek xénos stranger, guest (noun); alien, foreign, strange (adj.)
bestiality
1. brutish or beastly character or behavior; beastliness.
2. indulgence in beastlike appetites, instincts, impulses, etc.
3. an instance of bestial character or behavior.
4. sexual relations between a person and an animal; sodomy.
They are more alien than animal. Hell, they have more sapience than xenomorphs do. Arguably, dicking a xenomorph would be closer to bestiality. The only factor you're arguing for here is the equine-like appearance vs a totally alien one. Which'd be incorrect for the classification.
TL;DR: Alex right, you wrong.