Has to do with several factors, most notably two things:
1) Historically and/or currently discriminated or injured minorities.
No one has been persecuted or injured historically, or currently, for driving an SUV (rare exception aside, but not as a group). Think: Homosexuals, black people, women, etc.
2) Immutable traits that aren't changeable or by choice.
One can stop being an SUV driver. One cannot stop being gay, or black, or change their gender (though they possibly can change their sex, they count in this group).
There's lots of interplay, but that's where the line is, typically, around those two items.
I would say there is a spectrum of how mutable traits are, of course stopping being an SUV driver isn't that big of a deal, however I would say for example changing your religion on that spectrum is a lot closer to changing your sex than changing what kind of car you drive. There are also studies that speculate that ones propensity for religiosity has at least some limited genetical basis.
See above definition. You can be offended that people want to "murder babies," or offended that others want to "control your body," but being prolife or prochoice isn't typically a historically oppressed group, nor an immutable trait. So they need less protection.
What if those people feel the babies that are aborted are an oppressed group with an immutable trait in need of their protection the way many other people try to stand up for the rights of minorities that you agree need protection? I'm pro-choice myself, however your argument is on based on opinions on who constitutes as oppressed and in need of protection. Pro-Lifers opinions differ on this and feel the "babies" are being oppressed rather than the women in this situation.
It is tough. It's often a judgement call, and a difficult one, at that.
But can you see how sluring homosexuals is different than an athiest bashing on a Christian, or a Christian bashing on an atheist? Both have had some discrimination in the past in certain cases, but it's not a generally discriminated thing today, nor is it a trait anyone is born with.
In the end, the overriding #1 site rule is "don't be a jerk." That's to everyone, all the time.
Posting an opinion isn't being a jerk. Calling someone a name is. The athiest and christian mentioned above may cross that line, if bashing someone, or may not, if sharing their opinion in a polite way, even if it offends someone.
Don't be a jerk is a clearer line, and chances are, if you're offending people, you're being a jerk. We'll still allow you to say it, we'll just strike it out so that it's clear it's not acceptable*.
*Assuming we, mods, see it, or it's reported to us.
Except that atheists have historically been oppressed all around the world and in many places still do. In many African and middle eastern countries you can be put in to prison or put to death for admitting to be an atheist and in even more of them you will face those if you advocate for atheism the way others advocate for their religion. Even in many countries where atheists are tolerated they have to accept being a marginalized group, even in the US no politician wants to admit to being an atheist and any that do generally don't get very far. In polls about trustworthiness atheists rank below homosexuals in the US, so if it can be said that gays are oppressed then I fail to see how they end up ranking higher in public opinion than atheists do.
I would also say that being atheist isn't a choice for most. I can't force myself to believe something I don't. I could act as if I do to make things easier, but would that be any different than a gay or lesbian acting straight and hiding their true feelings?
Also kayvent covered this pretty well, someone can be oppressed for their characteristics in one situation and on the other hand someone with the same characteristics can be an oppressor elsewhere. A Christian in the US would be the majority and could be argued to be oppressing, homosexuals, non-christians, atheists etc. However Christians in muslim majority African, middle eastern eastern and asian countries are often oppressed and persecuted. This isn't even going in to who feels oppressed, despite Christians being the majority and often in control in the US many feel oppressed when held to the constitution and the amendments as far as separation of church and state goes by people from other religions or atheists. So in these kinds of situations you have a group that is effectively oppressing another group, yet feels like they are the oppressed group.
You say there is a difference when you have atheists bashing christians and christians bashing atheists and this might mean it is different, however I see plenty of LGBT people bashing back Christians and Conservatives for how they have been treated by those groups. Does this invalidate their feelings of oppression?
Jews have historically been an oppressed group, however I would say Israel can definitely be seen as oppressive by Palestinians. However, non-Israeli Jews in Palestinian territory are also often not treated well.
In my opinion you are acting as if your opinions on who is and isn't oppressed are not just opinions, but facts when the oppressor/oppressed dynamic can be very subjective and inconsistent.
In the end I feel that though oppressor and oppressed may change that we should always fight for as much freedom of speech because one day you might not be the one deciding which speech is and isn't acceptable.