Firstly, one is not, strictly speaking, born into a religion, although naturally the beliefs one encounters when growing up, especially those of one's family, exert a powerful influence. One can be born Jewish, of course, but that is a racial matter rather than a religious one - no-one is born into Judaism. So there is an element of personal choice here that is largely lacking in racial classification. (Only "largely", because there are many examples of people self-identifying with a certain racial grouping on census forms and so on.)
Secondly, and to my mind more pertinently, there is the matter of right and wrong. No-one in their right mind would say that it was somehow "better" to be black than white, or to be Indian than Pakistani. However, many religions' followers do say something very similar to this when it comes to belief. True, the average Christian would not say they were any better a person than the average atheist - but they might well say that it is better to be a Christian than an atheist. After all, if you feel that turning to Christ is the only way to become "saved", then by definition you must consider this the superior path.
But this argument goes further. The aforementioned Christian would believe that the atheistic view that there is no God is not only spiritually deficient, but factually wrong also. Conversely, the atheist believes that quite apart from being morally unnecessary to accept the existence of any higher power, there is no factual basis for the belief either. These two viewpoints cannot be reconciled, beyond saying, "We will agree to disagree". Saying such a thing would seem to most people to be the common-sense solution - but hard and fast law built on the shifting sands of human morality tends to be inimical to common sense. If we were to take a ban on religious discrimination and offence to its logical extreme, it would become illegal for a Muslim to state their beliefs in front of a Christian, since Islam does not accept the divinity of Jesus, seeing him instead as a human prophet in the line that ended with Muhammad. This contradicts a central tenet of Christian beliefs.
It is probably only fair of me to declare my hand here: I am an atheist. "Aha!" I can hear some of you saying, "No wonder he doesn't want these new laws, then!" But hold on a minute. Opinion polls in the United States over several decades have consistently shown that all other things being equal, Americans are less likely to vote for an atheist Presidential candidate than one from just about any other group - whether Christian, Muslim, black or (perhaps surprisingly) homosexual. This distinction is significantly less pronounced in Britain, but a low-level prejudice against atheists still exists; for example, in the largely unchallenged assumption by leading politicians that "faith-based education" is inherently better than secular schooling. Also, many public figures will spout what they see as inclusive rhetoric, stating that their vision for Britain (or whatever) is one for "people of all faiths" with little or no acknowledgement that millions of Britons have, in that sense of the word, no faith at all. So, provided that the proposed new laws also protected me from discrimination on grounds of lack of faith, I would personally gain from them. Even so, I cannot completely support the extension of anti-discrimination laws to religious matters.
All the existing legislation in this area - whether on grounds of race, sex, sexual orientation or age - relates to things which are not a matter of choice. Of course it is unacceptable for someone to make hateful remarks about another person because of the beliefs they hold, but to say that a freely-chosen standpoint should be somehow exempt from honest criticism is going too far. Surely the simply thing to do would be to abandon our current piecemeal approach, and simply introduce a new offence of "inciting hatred". But if I wish to say that I find parts of the Bible ridiculous, or someone wants to tell me that my atheism is silly and illogical, then the ability to do that must be protected. Without it, an essential component of a free society would have ben damaged, perhaps irreparably: the right openly and honestly to hold beliefs and opinions must be balanced by an equal right openly and honestly to take issue with those beliefs. The one absolutely requires the other.