On Mary's "special ability": I believe in it, completely.
I'm no bombshell -- and wasn't even when I was younger. But I can tell if a man is a "red blooded" man. No, it isn't automatic. But I can tell a joke, or look at him a certain way, and see if he responds, even a little.
This skill has a name -- flirting. Well, it's lighter than real flirting, since there's no promise of action in it, but it is of the same class.
I read those passages as Mary giving the man in question "the act", and seeing if there was a response. No response would imply the man was hag-ridden. And one of those "men" was a teenaged boy, who should have responded to any female under 65.
Not gay. The book was written in 1951. Homosexuality was not just in the closet, but wrapped in tissue, in a bag, in a box, buried under a pile of old clothes, in the closet.
This bit made perfect sense to me. And from what little is written about Mrs. V. Heinlein, she would have been a master (mistress?) at this act, and it would have been perfectly believable to her, too.
--Liz
http://www.printfection.com/cartesianbear