This is an argument that constantly gets thrown around by bisexual people I know, most likely because they assume others think the same way they do.  I can say, for my part, that so far in my 27 years of life, I've yet to have the desire to have sex with a man.  Perhaps Mr Right will come along tomorrow and sweep me off my feet, but based on my mountain of evidence, I'd have to conclude that I'm not attracted to other men.  Most of my straight friends have shown no real attraction to men as far as I can tell.

Likewise, my gay friends have shown know inclination for the opposite sex, which really makes this argument kind of silly.  There would be very little societal pressure for a gay person to fake not being attracted to the same sex.  For most gay people, being satisfied in a heterosexual relationship would make their lives easier.  Even if they were only 25% straight, they'd still likely pursue heterosexual relationships.

I can't find your profile for a direct talk so I'm gonna ask it here. Is your avatar suposed to be Frank, from the Percy Jackson/ Heroes of Olympus series?

People see it as the norm because they are accustomed to it. It's because it has been (unfortunately) taught for millennia as the only normal thing, and anything else is unacceptable. Religion plays a part, but it's mostly due to peer pressure and living up to the expectations of the people we know.

You are right. The support for what Zero999 said comes mainly from Kinsey Reports, a rather shady study from the 40's, and its derivatives, and a paper, can't remember its name, who claims to have measured the body response of men and women to sex scenes of various kinds, including animals.

While the first is flawed because of the apparent biases and lack of a good statistical methology, the second gauges only instinctive response to visual sexual stimuli without a care for the psychosomatic aspects of the people being studied. Because the study very easily leads to the unfortunate implication women and even children enjoy sexual contact because that's the way their body answers.

Of course, that's not to say sexuality is 100% clear cut... but again, nothing in life is. But when people use the existence of orange to deny red and yellow as separate entities, you know there's probably something wrong with the way they think...

You certainly enjoy your straw men, don't you? I never once implied that reproduction is the only reason for intercourse.

Think of it this way: Assume there are two identical species, one in which the majority of the species feels mostly heterosexual attraction, and one where the majority feels mostly some attraction that is not heterosexual. Which species is going to be much larger in number, and therefore, much more likely to remain in existence over time?

Attraction and pleasure are not the same thing. Dildos are not meant to aid attraction (on my own side note I am sure there are some lesbians who dislike the appearance of a cock enough to avoid realistic dildos, but generally lesbianism isn't about a hating cock)

"Just because it produces offspring" *facepalm*

Why is it the norm? This has to be the stupidest question I have ever heard. Yes, it is because a man and a woman are required to create a child. Just because it is the norm, this does not have anything got to do with hating on gay people. I would also like to add that I know plenty of gay guys who hate on straight women. Again, not relevant to hetrosexuality being the norm. In reality, questions like this are what cause problems. Stupid questions that divide people and highlight the difference between people. Questions that try to make it appear like if you think hetrosexuality is the norm, then you hate homosexuality.

Stupid!!

the argument that a penis fits in a vagina is just wrong
a penis also fits up the bum, in the mouth or in the ear hell it even fits in a hand

So either:
1. Your partners ear canals are huge
or
2. Your penis is tiny

Also "fitting" into a hand isn't much of a point, the hand has evolved as a multipurpose tool and lots of things can be said to fit it.