I shall address this topic in the form of an open letter to all women.
Note: I'm not going to discuss other permutations of male/female relationships, such as how they evolve, whether or not men and women can actually be only friends (they can, for as long as both can put up with the other being with someone else), and so on. I'm simply taking the one topic -- why a guy freaks out when their friend that is a girl dates some other guy -- and explaining it out.
As for whether or not this applies to all friendships with all men and women: No. It only applies in the situation stated above. If a woman has a guy friend, and starts dating another guy, and the guy friend turns into a jerk, the situation applies.
So, you have this guy friend. He's a good guy friend. He's funny and he's smart and he gets you and you can tell him anything. That's nice.
You begin dating a guy, and your guy friend turns into a big jerk. You're confused. "Why," you ask yourself, "would my friend suddenly turn on me? Isn't he happy for me that I've found someone I connect with and can have a relationship with?"
Well yeah, he was happy for you when it was him. Now it's someone else. You basically just cheated on him right in front of his face. Confused? Well don't be.
Your guy friend turned into a jerk because HE IS A BIG FAT PUSSY. And he's not your friend.
Harsh? I don't think so. As a former "guy friend" of a girl, I feel I'm in the appropriate place to call out every single "guy friend" who does this as being cowards. I am lucky enough to have married my best friend. But that's only because at some point, I fixed the problem I'm about to explain to you.
He's pissy because, while you saw him as great and wonderful and kind and smart and charming, you never took the step he was begging you to take. You never completed the dots and said "be mine." And it's your fault, in his eyes, because he doesn't get that you don't work like he does.
He poured effort into you. He showed you time and time again that he would care for you and nurture your every emotion and provide for you. He was so very sweet to you and was always there for you. Because he wanted you. "But he's not like that!" You're thinking.
Nope. He is. We all are. Period, end of story.
"But Joe, no, you just don't get it!" You're saying. "He said he wanted to just be friends!" No, no he did not. If you think about it, YOU put that out there. You may not have said it in those words, but at some point, somehow, you let it be known that you were very comfortable in the niche you've just carved out that you've labeled "friendship."
And because he's such a pussy, he was satisfied with that. He saw something in you he wanted to be around, and because he sensed that he might lose it if he was forward (or, was just too chicken to make a move), he settled for what he could get. He was so afraid of losing whatever bond with you that he had that he couldn't go the rest of the distance and let you know he wanted to be with you.
So, you found someone you found romantically interesting and your friend suddenly turned into a jerk. It's not jealousy and it's not thinking this guy isn't right for you... No guy besides HIM is right for you, in his eyes. And you chose someone else. That stings.
Men can (and will) pretend they're enlightened and beyond the animal needs of sex. In fact, they may even convince themselves it's true. If you ask them, they'll lie right to your face (and to their own) and say "no, I'm happy just being friends!"
But you introduced another man and they got angry. That should tell you everything you need to know. But it doesn't, does it? Because women CAN be emotionally connected and bond with other people without the concept of a committed relationship, while they see physical intimacy as something reserved for a mate (committed or not).
Men are the opposite. They invest emotion, but physical intimacy doesn't actually require that investment (for most men). If it sounds neanderthal and base, well... It is. We are animals, after all. And while it's true that there will be guy friends who don't freak out when you date someone else who still want to sleep with you (trust me... they do), the ones who do freak out are the ones who betray all their "nice guy" posturing.
I'm not saying you can't let a relationship build from friendship. But at some point, one of the two of you will come to the conclusion that you want more than what you're getting. You should just be up front about it. And when a man isn't, it's because he's a coward who is too afraid of being hurt and left alone and rejected to stand up and say "Look, I want this" and let the chips fall where they may. They don't realize that finding out you want something different than another person isn't the worst thing in the world. In fact, it's not even irrecoverable. It's difficult to hear at first, but it equalizes the situation and lets you both know where you stand, which is the only way that sound decisions can be made.
But yeah, that's why he turned into a jerk. Because he's a pussy. That's all.
Your internet guy friend,
And, having been a former "guy friend" in the past who finally grew a pair and figured out that being a spineless passive-aggressive wuss is no way to go through life, I figured I'd also include an open letter to all the "nice guys" out there:
Dear Nice Guy:
You're not a nice guy. Stop the bullshit. You're manipulating her. It doesn't matter that it's "nice" manipulation, it's manipulation. You're too coward to step up and ask her out, so you figure you'll win her over on "the things that matter."
You're a coward and a pussy. She's dating "jerks" because jerks step up and ask her out. They post a question which is answerable. It doesn't matter if she SHOULD be with him, she's with him because he gave her the opportunity to answer a question.
I get that you think you're doing the same thing but you aren't. You're not asking a question. You're setting a stage and hoping she acts how you want her to. That, my friend, is manipulation. And I hope to God hearing this makes you feel as dirty as I felt the day I finally realized it.
Trust me, life is SO MUCH EASIER AND BETTER when you stop with the games. The day I finally stepped up and told my best friend how I felt about her, she married me. Yours may not marry you, but how the hell are you going to ever find out if you don't give her the opportunity to work with data that can actually be quantified?
Also, that guy she's dating? He's probably not as big a jerk as you think. He's just not you. And you, my friend... You're really the jerk. Because you're the one jerking her around and trying to make her take the initiative so you don't' have to feel rejected. Think about it a little and decide if that's who you want to be. Either way, good luck.