Things like this could only be for sure answered after studies looking at the charts of typical heterosexual men and effeminate heterosexual men--but I'd say the Moon and Venus and signs like Pisces would have something to do with it and the ascendant; even Mercury since it copies whatever planet it contacts and is androgynous in nature.
I wouldn't say my astrotwin Zac Efron to have effeminate traits, I don't know him personally, but he is effeminate
looking and has a first house Venus conjunct Pluto in Scorpio, with Moon sextiling from Virgo. Mercury is conjunct Venus as well and with Moon&Mercury being convertible this gives him a strikingly effeminate face--I think I can see a bit of the 'trickster' Mercury in his eyebrows. He also has Venus sextile Neptune, and Moon trine Neptune very closely.
His chart ruler Mars is in Libra, as is the Sun. So his masculine planets are a bit effeminate as well (both under Venus rulership).
I know this thread is about traits, but it'd be interesting to know how his mannerisms are as well. I would expect those to also be effeminate if he isn't hiding it to not appear gay. He's been teased about that in the media.
In my opinion those with feminine traits are going to have strong contacts to Venus/Moon in some way especially if they contact Neptune (but I wouldn't say Neptune alone).
Honestly, I think it's extremely difficult to have this discussion these days because the weakening and softening of American males through school and media culture has been extreme in the last 30 years.
Most notably as far as this picture above is concerned is the gay influence. Gays are most of the voices and minds behind fashion and all related things and I have personally watched gay styles become "the accepted" style for male youths over the last 15-20 years, just through media saturation which make "trends". Their is a gay mentality and perspective. They write about it all the time but the people don't think of it that way. Check out the book "Boi Culture."
The alternative to the above look is the "thug" look, which takes its inspiration from prisoners, most notably the style of wearing the pants far below the buttocks. This comes from the prison culture where it's a symbol of a submissive, obedient, emasculated male. The rest of the style looks borderline homeless and always juvenile.
The basic creed is for boys to look, act and think like Boys for as long as possible. You see this in fashion, you see it in the behavior of 40 year old men still trying to live out their 16 year old fantasies. I find this entire pattern disgraceful in the extreme.
Becoming a Man is barely a concept these days, let alone something one pursues, prepares for, works towards, is trained how to do- internally and externally.
If you put this exact same boy, same chart, in the 1930s in Virginia, he would think, look and act a completely different way- namely, he would have been learning how to be a man and physically doing things that come from and enhance male essence, form and function- from work to dress to speech and life goals.
Today we have superficial words trying to describe far deeper forces. Problem is these words are misshaping and separating consciousness from the deeper, true forces of Life.
One becomes a Man. One is not born that way. In the process one learns how to express the energy of the stars through one's Male Essence and Form. Essence leads, Form and Function follow.
For example, sensitivity has nothing to do with "effeminate" if you understand the deeper meaning of "sensitive" as meaning "aware".
By the same token, a man who expresses his heart is not "in touch with his feminine side." He is HONEST and unafraid of the consequences, good or bad. Honesty has nothing to do with feminine or masculine, nor does the Heart.
But these words and phrases litter the American consciousness and do nothing but obscure and warp people's minds and views. And as we know from the latest research in quantum physics, words and thoughts shape reality, in any direction.
It's all very unfortunate and often disgraceful. And I should add, to me the word Man automatically includes the words Wife and Children, Husband and Father. They are built in to the word Man. I do acknowledge some exceptions of course, such as the extremes of warrior and artist that often make wife and children impossible. And of course the monks who remove themselves from the cycles and rhythms and dance of Life entirely.
But these are extremes, like Nicola Tesla and Duke Ellington or the Dalai Lama or any number of samurai during the warring states period in Japan or similar warriors in the ancient Mediterranean world.
He has Virgo Sun, Cancer Moon and Saturn square Uranus in 12th house. He's shy, loves colour and he understands me better than some of my women friends ! 
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