Conceid

Imagine, if you will, that you're a straight man.  It's easy for me to imagine, of course, and there's a reasonably good chance (somewhere around 43%) that it'll be somewhat natural for you to imagine as well. 

As a straight man, you know one very important thing about sexuality: women are hot.  Not all women, necessarily, but many.  There are aspects that you might use to identify a woman that you would also identify as being attractive to you: breasts, slender legs, diminutive features when compared to an adult man, long hair, large eyes, high cheekbones, red lips or cheeks, and so on.  Your specific definition might be unique to you (perhaps you like women with mohawks, or women who've had amputations; I don't know), but you'll likely identify with several "standard" ideas of what's attractive in a woman.  We agree so well on these that scientists have produced ideal ratios of attractiveness (If you measure the distance from the top of a woman's head to the bottom, the distance between her eyes and lips should be 36% of that first distance for optimal attractiveness; similarly the distance between eyes is 46% of total width - Link).

It's easy, as a straight man, to see what other straight men see in a beautiful woman.  It is also easy to understand what a gay woman might see in another woman.  It is quite easy for us to understand homosexuality in women, even if we might dislike it for fear of its extracting women from the pool of eligible mates (yes, that's how I actually thought back in my older, dumber days).

But homosexuality in males is another matter.  It's difficult for a straight man to see another man as sexually attractive, even if the other man is physically attractive - that is, I can readily admit to the physical attractiveness of your Brad Pitts and George Clooneys of the world, but difficult if not impossible to translate that understanding into an understanding of someone's sexual desire for them. 

As a result, we deny their physical sexuality by virtue of their attractiveness, and ascribe sexuality to other factors - fame, money, prestige, strength, masculinity, and so on.  We assume that a woman who is interested in them must be interested in them for one of those reasons (aka, "gold diggers") rather than physical attractiveness. 

We take this even further when trying to understand homosexual men's attraction to another man, and assume either femininity on the part of the attractee - that is, the more masculine male in the relationship is attracted to the more feminine male because of his femininity; the more feminine male is projecting his own femininity by searching for a more masculine partner.  (This is the assumption behind the "pitcher and catcher" cliche.)  When seeing a gay man alone, who does not possess any obvious (read: stereotypical) gay traits, we may search for other causes, such as a missing father figure (the need for non-sexual male bonding being mistaken for sexual male bonding). 

Regardless, the reason can never be simple physical attraction.

And, as you can hopefully see, these are exactly the points we all generally agree with - that is, unless we know we have things incorrect and have investigated them to draw them out, we take these points at face value, and they drive a lot of our conversations about each other.

Which is a backwards way of saying that everything we think about sexuality and what is attractive and what assigned roles people are filling within this complex story are all directly derived from that initial premise: that the actor and subject creating these definitions is a straight male.