Since when has “wallflower” been used to refer to men?

Dictionaries these days define wallflower as a shy or unpopular person not dancing at a party (see Merriam-Webster for instance). Etymonline says the first recorded use of the word in this sense was in 1820, but it restricts it to women:

Colloquial sense of "woman who sits by the wall at parties, often for want of a partner" is first recorded 1820.

The Kipling Society also appears to suggest that in the 1880s wallflower in this sense applied to women only. They explain the use of wall-prop (this is the subject of another question) in Kipling’s “A Friend’s Friend”, Plain Tales from the Hills, 1888:

wall-prop a non-dancer who leans against the wall. (A non-dancing lady was a 'wallflower').

The wall-prop in the story is a man. So, did wallflower in the sense of a non-dancer initially apply to women only? If so when did it start being applied to men as well?

In response to Jacinto's comment, I searched again and found a reference to a man being a wallflower, in which it was not so gender-specific, although in the context it is already known that they are talking about a man. In 1913, Corra Harris wrote In Search of a Husband. On pages 47-48 I found the following passage of conversation between two women:

"Still it was queer, not to introduce him," I insisted

"Not queer, merely impudent. But he had the best of it. The difference between being a wallflower and an indifferent celebrity upon such as occasion is accomplished by turning your back to the company. Wallflowers sit or stand face forward, confessing defeat. He turned his back on us and put us out of countenance."

I've located a couple of other early 20th century references to male wallflowers, which are much more gender-specific.

From 1910, in At the home plate, by Albertus T. Dudley, page 123:

"I wonder what kind of a kid Crusty was when he was at school!" he thought, as he staggered back to his seat. "I'll bet he was a wallflower. It's queer that he should have a brother who can play football."

From March 1918, in The Recruit. A pictorial naval magazine. v.4 no.3 at the end of the first paragraph of page 24, from a short story called "The Wall Flower":

He was a wallflower

When there was a happy crowd grouped enthusiastically about the piano in one of the girls' homes, singing, laughing, and at ease, where was he? Sticking out as solemn and aloof as a sore thumb, that's where.

Further back in 1884/5, in The Freemason's repository. v.14 (following a list of questions regarding active participation in the Freemasonry), on page 133:

Is it not a lamentable fact that in the great majority of instances these questions would have to be answered in the negative? And now, by virtue of his office, he is entitles to a seat in the Grand Lodge. Is it any wonder he is a wall-flower there. Is it strange that the business of the Craft falls into the hands of a few men, who at the sessions of the Grand Bodies are overwhelmed with work?

It was originally used only for women, from the OED:

• colloq. A lady who keeps her seat at the side of a room during dancing, whether because she cannot find a partner or by her own choice.

  • 1820 Praed County Ball 148 The maiden wall-flowers of the room.
  • 1840 New Monthly Mag. LIX. 340 He..dances quadrilles with every wall-flower in the room.

My personal impression is that it has become common referring to males from the Disco era of the 70's.

Wallflower:

• It was originally used to refer to women, and only in the context of dances; more recently the term has been expanded to include men and other social gatherings. (Wikipedia)

Ngram: male wallflower appears to be used mainly from the 70's, but the are earlier instances.