If a straight woman with a gay man is a beard, what do you call a straight guy with a lesbian?
Hard up. 
…Is she “butch” or “lipstick”?
I was under the impression that a beard was a male friend covering for the fact that you have a mistress. By taking her out and being seen with her no one is the wiser to who is dillying whose dally.
Of course I’ve been wrong twice before…
In my experience it’d be rare to hear someone refer to a straight woman out with a gay man as a beard. Fag-hag maybe - awful expression!
Monkeymensch is right I think. The beard is the “cover” to confuse observers. Kind of the way a chaperone is supposed to ensure that everyone’s reputations stay spotless.
None of the above have worked since 1960.
Redboss
According to this Glossary of Sayings, Slang, Signs and Symbols for Gay Men, I think the answer to the question in the OP would be “a beard”.
If facial hair on the chin and of a man is called a beard, what do you call facial hair on the chin of a woman?
happy.
I have on a couple of occasions had the responsibility of escorting lesbian friends who were not completely out of the closet.
We were under the impression that I was their beard.
A gay friend of mine recently grew an actual beard, which he has named “Sophie,” after the wife of Prince Edward (aka “Dockside Doris”).
I think most of the usages mentioned are actually correct.
The 4th definition in the AHD is simply “One who serves to divert suspicion or attention from another”, and I’ve heard it used in several contexts, not always sexual.
The “cover” etymology seems natural, whatever the genders of the coverers and coverees.