I read on the Heinlein Facebook Group:

Heinlein Forum
David Lubkin ·
6d
·
Heinlein used to refer to a man's involuntary physical reaction to an
appealing woman as the "gallant reflex."
He was riffing on the name of an actual response, the Galant reflex,
named for neurologist Johann Galant. It's checked in assessing newborns
for brain damage. Also known as the truncal incurvation reflex. Hold the
newborn upside-down and stroke along one side of the spine. You should
see the baby flex toward the side you're stroking.

George Schmidt
I think when RAH used gallant reflex, he was taking about getting an
erection.

David Lubkin
Author
Of course. I was just not saying the word.

George Schmidt
David LubkinI read your first sentence differently the first time. My bad.

John Tilden
Admin
Well, you knew it would come up.... 😉

Sandra J. Collier
Lololol. That's SO funny!
"Is he alive?"
"I dunno, show him this photo of Betty Grable."

John Powers
Sandra J. Collier Oh God above, that had me gasping for breath as I
laughed!! 😂🤣

Keith H Weston
Sandra J. Collier You really should read Stephen King's 'Autopsy Room
Four', if you haven't 🙂

David Lubkin
Author
The two phrases have an interesting history.
No photo description available.

David Lubkin
Author
Also, gallant and Galant aren't entirely unrelated. It appears that if
when the baby is upside-down, you stroke 𝑏𝑜𝑡ℎ sides of the spine,
they will involuntarily pee. The reflex is ordinarily gone after the
first year but when it is retained or returned, can be a cause of
bedwetting since body movement in sleep, especially under covers, can
provide the reflexive stimulus.

John Macgregor
Some just say “heavy petting.”

John Hanson
I've always called it a boner.

Mike Galos
John Hanson That's why we read Heinlein and not Hanson.

David Lubkin
Author
Mike, took me a second to realize you meant the guy you were replying
to—at first I thought you meant the boy-band brothers from when my
daughter was a tween, who sang "Mmmbop, ba duba dop." We certainly don't
read them.

Michelle Malone
David Lubkin Uggh.. you unlocked a memory I wish you hadn’t! My friend
and I got their single (no need to say title- there was only one hit),
and we played it about 200 times in a row