Umm... I'd say only atheists can have meaningful moral opinions. "Because god said it is a sin" is not a meaningful opinion. Religion takes the meaning out of morality. When religious people are thoughtfully moral, it is usually in spite of their religious belief, not because of it.

Arguing that something is wrong because it is a "sin," as David Foster Wallace once wrote in a different context, is like arguing that murder is wrong because it is against the law. It's a circular, deeply superficial and content-free argument. You merely substitute one word for another ("sin" for "wrong").

To the extent that the religious go beyond "god said it is a sin" they are actually appealing to fundamentally atheist moral arguments. For example, "because it causes pain to others" is actually an atheist argument, not a religious one. When religious people use it in some form ("god said it is a sin, because it causes pain") they are bolstering the weakness in the divine notion of sin. The god clause in the above example is actually superfluous. "Because it causes pain" has the same content.

The only way religious people can make their arguments deeper without using atheist reasoning is to appeal to the afterlife and some sort of calculus of beyond life reward and punishment. In my opinion this is actually a lousy way to deepen the argument (even ignoring the fact that there is no reason to believe it to be true) because it places notions of wrong and morality into an extrinsic motivation framework. You should not kill or rape because even if you get away, you'll be punished with eternal hellfire? Really? That's the best you can come up with? You should be kind to the unfortunate because you'll get booze and women in heaven? Really? That's "meaningful" morality? It's the reverse. It empties behavior of even its natural moral meaning. Even without reading deep ethics tomes, a small child will be kind to a crying friend. Putting religion in that child's head is equivalent to taking that natural unreasoning empathy out.

How do you actually make the argument deeper (for instance by asking, "why is causing pain wrong?")? You have to go where no truly religious people will go. To subjects like evolution.

For instance, we can understand "pain" through evolutionary notions of empathy, cooperation and reciprocal trust and get to a point in the analysis where we don't even need the word "wrong." Only within a Darwinian framework can we even talk about what the existence of rape among dolphins implies for the corresponding human behavior, because we understand at some level that our model has to cover both cases (since there is no artificial distinction elevating man above animals for atheists).

This allows atheists to analyze and apply the "don't cause pain" in much deeper ways than religious people. For example the religious usually make humans out to be special and don't usually care much for animal pain. In fact, one religious guy I know argued, "but God put the animals on the earth for us to eat!" What do you get as a result? Factory farming. A military-industrial complex of horrendous pain-creation justified by some notion of humans having dominion over animals by divine sanction. Yes, I assign a significant proportion of the blame for things like factory farming to religious notions of morality.

The religious are also less likely to understand and apply practical notions of deterrent pain and more likely to cause more pain than necessary. The religious might believe in some notion of divine symbolism and symmetry (an eye for an eye, or cutting off the hand of a thief, or stoning an adulterer or pretending that killing in war and capital punishment are somehow different from murder) in their pain calculus. The atheist is much more likely to see that pain delivered by the criminal justice system needs to be applied with reason (you put a hungry street orphan who steals bread into an orphanage and FEED him; only a religious monster would cut off his hand) and at the bare minimum level required to serve as a deterrent to prevent future crimes of the same sort.

In fact the very word "justice" is a terrible word, pregnant with religious meaning. If we renamed the "criminal justice system" the "crime deterrence system" it would become a good deal more sane overnight. For starters, we'd be more willing to frame questions like sky-high incarceration rates of black males in smart ways. A "justice" framing automatically applies the divinely stupid logic that "crimes" must be "punished" in proportion to their severity. It is a notion of societal revenge (on behalf of the victim in some cases) legitimized by some notion of god and sin. A deterrence-centric system automatically assumes that the goal is to minimize violent crime and find deterrents that actually work and don't punish individuals for systemic ills created by historical forces.

"Social justice" is an even worse religious-morality framework, but we won't go there.

A particularly tough one is the abortion/euthanesia pair. I am neither pro or anti-choice in either of those cases. I think it is a situation-dependent tradeoff among competing factors, where the responsible thing to do is to promote life where possible, but be willing to make the calculations if necessary and abort/not abort euthanize/not-euthanize depending on the situation.

But the abortion and euthanesia debates are deeply muddied up by bringing religious morality to bear, because it shifts the debate towards thought-stopping absolutes and usually causes untold real pain in the name of avoiding abstract "wrongs." In a way, "pro-choice" is language that shows that the religionists have won these debates. The real dividing line is between "pro-thought/anti-thought." The "pro-choice" foundation of right over one's own body is deeply limited in how far it can go, and will always lose to the "pro life" framing for abortion and euthanasia.

Religion stops such thoughtful, deep morality in its tracks by adopting absolutist stances where thinking is needed.

Ultimately, to be moral is to be thoughtful. To be thoughtful is to be willing to think as hard as you can to make decisions. Sometimes you can think all the way to an answer that leaves you no real choice. Sometimes, you are left with a residue of unknowns and you have to make a choice guided by some heuristic such as "first, do no harm."

But in either case, unexamined religion usually stops thought in its tracks.

Added: Am adding here an additional point based on a comment by Marcus Geduld that religious people invest their notion of god with "authority" in the sense of being more knowledgeable/expert than themselves.