In my experience, there are three types of gamers.  Those who have a preference for a particular console (because we all have a preference), those who support a particular brand, and finally there are the fanboys and fangirls.  For the latter, I offer a simple solution that will allow you to enjoy the other systems without contradicting your own unwavering bias. 

Purchase competing consoles and games used.  Go on EBay, pawn shops, sift through the Used shelves and bins at your local gaming store.  Consider the following facts:

1. Publishers and companies do not receive anything for used sales.  Purchasing a used console financially supports whatever institution you got it from.  By purchasing them used, you can feel guilt-free when playing another console, knowing that instead of supporting that particular brand, your purchases actually harm them in a very slight manner.

2. Your opinion is more credible!  Rather than being in a position where you're simply spewing out unfounded hatred for other consoles, you can always refer to your collection and boldly (and more accurately) declare that your opinion is founded on your personal experience rather than brand loyalty.  This little detail will actually make your arguments more convincing, and you might be able to prevent your friends from purchasing a new one and supporting the brand(s) you don't like.  Victory for the fanboys and fangirls!!

3. You get a richer, more fulfilling gaming experience.  Face it, sometimes we don't like a console exclusive simply because it is for a console we don't like.  But if it was on one we do support, we'd be going stark, raving mad with anticipation.  This way, you can enjoy them.  You get to be true to your gaming tastes and not compromise your gaming library with questionable titles, and since you're doing your part to harm that console while you're doing so, the experience is that much more sweet.  Not only that, but now you have the ability to choose the superior of the multiplatform titles, because sometimes, one console gets sweeter bells and whistles than the others.

Bzzzt.

There is something wrong with your idea.

Firstly, to buy used consoles/games suggests that there was an original sale to begin with, to acknowledge the competitor is selling is blasphemy! (Which is what 360 fans had been doing all of last year on the PS3).

Secondly, knowing a used market exists, people will buy consoles first hand as they know they are able to sell them later on and recoup most of their money (in some cases, ALL of their money. Cough Wii Cough). In which case you may as well hand your cash to the companies.

My advice, ignore the fanboys/fangirls, by definition, you cannot convince them to change their ways, or else they'd be just another bias consumer and not a fanboy. A fanboy will never be swayed, even if the overwhelming odds suggest they should and sometimes, they are rewarded for that (Nintendo), other times, they suffer (Sega).

I agree with your first paragraph entirely. I think that some people have no idea how biased they are. This is the first gen in which I've owned all the major consoles and it really makes you realize how pathetic some people act.

I thought this was going to be a two word OP, as a solution for fanboys... something along the lines of:

"Shoot yourself."

Yeah I wanted a zing.

You're not going to cure my Nintendo fetish.

They've had me hooked since the D-Pad.

The solution for fangirls is to send Mr.Y yer digitz.