I see what you were trying to do, but...when you have a preconceived notion that people are going to treat you differently because you're a certain gender, you're more likely to view the results in favor of that notion. When in reality, most people probably treated you differently because your gender suddenly changed, not because you were suddenly female. You were previously established, by yourself, as a male, and then suddenly that "fact" that was previously established, changed. Behaviors can change as a result of that. Not because gender itself, because change. You might have thought they were acting nicer when they were acting just as nice as they usually do.
People do have a tendency to change present and future behaviors when a fact that had been established as true suddenly becomes false. These results can't really be used to make a statement about different genders being treated differently because without specific examples, and since the target "audience" was already led to believe you were a male, by yourself. Not saying genders are treated equally on the internet, just that this test doesn't really showcase that. If you established yourself as female from the start of your membership, then changed to male, you would probably receive a similar change in behavior. On this forum, there've been plenty of people that have been judged as one gender simply because of one's avatar.
It became "socially acceptable" to judge other users based on the gender they chose for themselves on their profile. When you decide to just change that, people are going to notice and treat you differently because that's the equivalent of "changing genders." People will either think you're trolling / messing around, that you were lying about your gender the whole time, or that you decided to change genders or physically / mentally identify as the opposite gender from then on. So yeah, people might treat you differently for 2 out of 3 of those reasons.
I don't think this website has a big stigma about treating genders differently, like other websites mentioned by W1K. But honestly, this experiment seemed to be making a bigger deal out of different genders than the point it was trying to prove, if that makes sense.
To explain each point in a bit more detail.
1: People judge genders by avatars a lot. It's more acceptable to judge genders based on the gender option as indicated at the profile. When you change this option, people notice and comment on it. This isn't a surprise.
2A: You wanted to prove this, but in the process you kind of caused a bit of contamination to the experiment itself.
2B: Will people change their behavior towards you because you were now female? Maybe. Will it change because you were now female? Probably. The bolded words are the key. You established yourself as one gender and then suddenly changed. That's what likely caused behavior changes.
3: Most people were surprised that you were now female and confused. Again, 2B. "It feels strange that people would be astonished by someone not being the gender they expected." They expected that because they were led to believe that, and it was a fact established by you. They were likely astonished that it changed, it's not like you were an unspecified gender and then suddenly came out as female. You said you were male and then suddenly you changed it to female.
3B: Yes, definitely placebo action going on. You thought they'd treat you differently if you did it, and you wanted them to treat you differently for sake of the experiment and to prove a point.'
4: Without acting differently, or acting like a girl, people changed their behaviors towards you. They changed their behaviors simply because your gender changed to a female in the profile. It's not like you were acting girly and people were treating you great because you acted girly. You acted identical, whether male or female. They weren't treating you differently because what they thought you were. They weren't treating you differently because you acted differently. They probably treated you differently because you identified yourself differently. From the people that have posted in the thread, none of them seem particularly shocked, or like they were betrayed by your experiment, leading me to believe it's not like they were trying to get your affection / special attention when they thought you were a female. (Which, again, without specific examples it's hard to judge exactly how they treated you differently.)
5: Again, I understand what you were going for and I respect that you went for it. But...you're kind of turning oranges into apple juice here. Yes, obviously "gender doesn't matter." (Should really be changed to People shouldn't be treated differently based on their gender. There are people who take gender identification very seriously and it does matter to them, even on the internet.) But you can't really draw that conclusion from this experiment. You weren't acting boyish, or girly, so how would that equate to people treating you differently based on how you act as a gender? It really only leaves that you were being treated differently because, in their eyes, you identified yourself as the opposite gender. It wasn't surprise because you were a girl, it was surprise because you changed into a girl. Which you did because you expected to be treated differently, even if you didn't act different. Again, it's kind of placebo, and a self-fulfilling prophecy.
Not trying to be nitpicky or anything. Just saying that most people probably responded to you changing your gender, rather than the gender itself. I spent my early internet years not really "outing" my gender, and other times pretending to be a male, so I know how people can treat others based on gender. Especially when you come out in voice chat and everyone throws a fit because you were a girl the whole time, blah blah blah.
Also, I'm not saying the experiment was bad. Just saying it proved how people can respond to small details such as changing one's gender on a profile page. Even if you don't act differently, they treat you differently, however slightly. And they assume other things because you don't tell them, etc. So it's still a pretty informative experiment, it just can't really be used to adequately measure gender bias is all. Hope I didn't offend you.
(Also, yes I noticed the gender change and took interest in this situation because I enjoy to observe people to see what they do in certain situations.)