'Clean' is a relative term. The fact is, that healthy skin,
even recently cleaned, will produce natural oils, and, in
the sense that these oils on a halogen bulb will shorten its
lifespan, yes, they can ruin it.
Unfortunately, I could not find an exact explanation of why
this is so, and I'm hoping you'll feel that such an explanation
falls outside of the scope of your question, since a fair amount
of searching didn't turn it up. It appears to have something to
do with the fact that halogen bulbs are made from quartz rather
than glass, or silicon, as are incandescent bulbs.
The most comprehensive answer I could find was on the
TekLight site, which manufactures halogen bulbs for
medical equipment, such as lighted microscopes:
"Why shouldn't I touch my halogen bulb with my fingers?"
"Do not touch the surface of a quartz halogen bulb with
bare fingers, as the quartz surface will be exposed to
the salts and oils in the hand. This will cause a weak
area and lead to premature lamp failure. If you happen
to contact the bulb with your fingers, carefully clean
the quartz surface with a gentle solvent such as alchohol."
Other sites basically repeat this precaution, with even
less of an explanation as to the dynamics. These sites
can be found in the search results I've provided below.
These sites include:
"Do not touch the Halogen bulb surface or inside reflectors
with your bare hands. Oils from skin can lead to breakage
or shorten the life of the lamp. Use clean gloves or lint
free cloth for installation and removal."
"Bulb Care: Do not handle the glass part of a halogen bulb.
The oil from your hands or any other residue on the glass
can cause the bulb to burn out prematurely. If you
accidentally touch the glass clean it immediately with
isopropyl alcohol and a lint-free cloth."
Page 10 of this pdf file from Helios, Proteus & Apollo
"Do not touch the new bulb with your fingers; using a
cloth to hold the bulb, gently push it into the socket.
If there is any doubt that the bulb may have been
contaminated by your touch, you should clean the bulb
before its first use. Take a clean cloth dipped in a
small amount of rubbing/isopropyl alcohol and gently
brush off the bulb surface."
And, finally, and directly to the point of your question,
"Touching a halogen bulb with your bare hands will ruin it."
Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog
established through the "Request for Clarification" process.
Searches done, via Google:
touch ruin "halogen bulb"
touch oil "halogen bulb"
+why not touch "halogen bulb"