Here's one thing I want to put forth. Guys say that they don't understand women. Women say that they don't understand guys. Bullshit.
People don't understand people. The only thing is, if you're not in a deep intimate relationship with someone, you don't have nearly as much riding on the relationship, so you don't NEED to know as much. Guys can "hang out", and not really connect all that deeply. Girls can do the same. But when you're in love, there's a lot of tip-toeing and anxiety that goes on, trying to balance each others prolonged happiness, sexual gratification, etc. That's where the communicate becomes important. If two straight men, as friends, had to have that kind of tight communication, they would have the same problems. Just listen to roommate stories, even among best friends.
Someone from our resident queer community would probably be better suited for highlighting this issue, but I noticed that it is pretty all-encompassing. Next time I hear the, "Man are from Mars, Women are from Venus" comment, I just want to bash their head in.
That whole "Women are like a different species" thing annoys me as well. Sometimes I think people purposefully try to make the opposite gender some sort of enigma so that they don't have to learn anything about them, or feel justified when they fuck up. Relationships are hard at times, and excuses like that are just lazy.
I know I've had problems with boyfriends, almost identically similar to the kind of arguments straight couples have. I get a lot of laughs when I hear guys saying stuff like "It must be so much easier going out with another dude, you can just do whatever, and it's cool cause you are both guys." Eh, no. As Prime Mover said, it is a totally different situation when you start a serious relationship with someone, no matter how good of a "bud" they may be.
Funnily enough, I do actually have a pathetically co-dependent relationship with a straight guy. We have known each other for years and years and years (which is about 8, for those who like numbers). He is actually pretty much the perfect boyfriend, only we don't have sex. It's a very...very weird relationship.
As for what Alisha says, I get the same thing with some girl friends. After a while they stop treating me like a guy who likes guys, and more like a eunuch. But that's really here nor there, as any female friends I have are tomboys. I'm very much at home around male company, rather than a lot of gay guys who surround themselves with women.