I think the main qualm against space marines (And I'd like to preface this by saying I play
BA and we are very
op) is that they are
GW's favored child. They are the noob tube of
40k as it were. It's considered the sign of an inexperienced or a
waac sort of player now, because the space marines are always up to date, are always in the top teir in one form or another, and thus always have an advantage over non
SM armies. Plus, while most other armies get only 1 codex (even armies as diverse as
IG, with dozens upon millions upon trillions of worlds with unique armies and fighting styles) the space marines have Dark Angels, Grey Knights, Vanilla Marines (with
HQs to play as other chapters), Blood Angels, Space Wolves, and Black Templars. So in essence they have almost as many codecies just for their different chapters as there are other armies in the game. Because of all this, there are always going to be
SM armies at any tournament, and at any game store (
FLGS or
GW direct) because they're just so common...while other armies may not appear even after they get a new codex (I am still the only Tau player at my
FLGS...). So many players tire of being constantly bombarded with
SM armies, and grow to dislike people who play them out of a sense of frustration.
That said, there is absolutely nothing wrong with playing space marines. In fact, pretty much anyone who has played
40k has played as a space marine army at one point or another, be it testing assault on black reach, the battle of macragge, a friendly game, or what have you. So all those people who are berating you and belittling you are likely space marine players or have been in the past, and so have no grounds to insult you because you wanted to play an army you like.
The one exception to this (the exception that proves the rule if you will) is someone who plays
SM because he's
TFG. You know, the one who has to win, and then rub it in your face when he does. Then people have a right to be cross with you (royal you) for playing
SM. But I doubt you play like that so it probably doesn't apply here