Nobody is trying to show off. And it wasn't sexist as such, but doing the online equivalent of wandering into a room and going "I know about this thing, does anyone want to know about this thing, because I know. You can ask me anything." is just bizarre and odd.
Firstly it assumes that people don't know this already and are too stupid, too lazy or otherwise unable to find out such information for themselves. I mean if you use the search function on the site or even just look around for a few minutes, you'll find that there are many knowledgeable posters here, a tech forum, and lots of conversations about internet safety. Perhaps not on this board itself, but knowing the site, this is a less well known board with less traffic and most people would post questions of that nature in Chat, AIBU, Parenting or Geeky Stuff. TBH until now I didn't even know this section existed.
Secondly it ignores polite convention (in real life and online) about waiting to be asked or for a general question to be put out before you unleash advice.
But mostly it's rude because of the implied assumption that your audience is ignorant. Unfortunately for you, you (most likely unconsciously, given your reaction) happened to overlap with a common stereotype: That women aren't as computer-savvy as men, and that anyone over 30 is totally bamboozled by the internet. Even if you did do it unconsciously, you put these factors in your OP by pointing out that you are male and mid twenties.
(I have to say though, I just shouted out loud "Oh my GOD it's scottishmummy!" and forgot that it's 2am
)
Oh shit it's 2.30. I should go to bed.