The greed thing sounds like so much bullshit. If you're single, what does it matter if you go for a guy one time, and a girl the next? You're ALL available (or rather, should be, you naughty cheaters) at the end of the day anyway; gender is neither here nor there. You don't see guys going round claiming lesbians are stealing all their women. The only POSSIBLE way it could ever be greed is if bisexuals date them all at the same time, and last time I checked, bisexuality isn't the same as polygamy.
Honestly, if anyone ever comes out with that reason, I instantly judge them to be stupid, jealous, or both. No offense meant by that, but I can't believe people can't see through that kind of sheer ignorance, and surely greed would not be a problem if you could attract partners in the first place.
Jeez, I'm not even bisexual and I'm getting worked up over this. >_> I have absolutely no problem with it, apart from kids claiming to be bi because it's cool. Then it's just sad.