Darwinistic theories aside, perhaps there are reasons for such double standards. There are numerous dimentions to look at this as well. Perhaps, this apparent homophobia in some men is a result of latent misogyny. Misogyny does not necessarily imply the hatred of women; if men truly hated women, then most males would be homosexual. It is, rather, the views that imply that females are inferior to males that defines misogyny. This reasoning is quite simple: a man sees a homosexual; homosexual men are romantically attracted to males, as with heterosexual females. Since the homophobic person views "females" as inferior, the person will most likely view those who are similar to females as inferior as well.
Also, somewhat related to the misogynistic aspects of homophobia could be the various insecurities of men, in regards to their masculinity. Men who are insecure about their masculinity tend to have the psychological need to prove to themselves how "masculine" they could be. In other words, they deny that anything "feminine" is within them; if they admit to doing something that is regarded as feminine, or that they admit that they are capable of being feminine, they would be emotionally devastated. But what these men truly fear and loathe isn't really the act of being perceived as feminine, per se. It's the fear of being someone who they've perceived to be weak and inferior.
As a truly fascinating subject on gender, I have chosen to elaborate on the writer F. Scott Fitzgerald. In her book, Love and Death in the American Novel, Leslie Fiedel has said, "In Fitzgerald's world, the distinction between sexes is fluid and shifting, precisely because he has transposed the mythic roles and values of male and female...Thematically, archetypally even such chief male protagonists as Gatsby and Dick Diver are females." (312) What exactly does this mean, then? It seems that Fitzgerald's works and characters, regarding gender, are not subject to rigid boundaries and distinctions. Many of his male characters are depicted as being well-dressed, yet no characters assume anything about gender identity or sexuality in those instances. His male characters form close, intimate friendships with other males, contrary to most males. On the other hand, Fitzgerald's female characters are quite assertive and autonomous. His "mental and emotional androgyny", as I shall call, has created various androgynous characters as well. With only a few changes, his male characters could easily become females, and his female characters could transform into male characters with no difficulty. Most men, especially men who have either denied their feminine half, or men who are unaware of their feminine side, do not usually "gush" over a boy or a girl when they are in love. At least, they do not do so in a platonic, romantic manner, how Fitzgerald's male characters and females in general, tend to do. Men who accept their androgyny and females, are not afraid to associate in a friendly manner towards the same sex.
So, dear reader, throw away all judgements based on gender, and free your mind. Perhaps you may see things you never have realised before.
---Source---
Fiedel, Leslie. "Love and Death in the American Novel" (NY: Dell, 1960)