True, there are people who will be homophobic regardless of whether religion is there as an excuse or not, but I would still argue there are people who are are 'homophobic' as a result of religion and I think the 'belief in God' argument does a pretty good job of illustrating this.. (Of course, this depends on what 'homophobic' actually is.. see below)
Belief in God doesn't predicate being homophobic.
I wouldn't claim to know a great deal about religion, certainly nowhere near your level of knowledge but is it not thought to be a Sin in the Catholic church to engage in homosexual sex? I'd argue that saying they're doing wrong or judging someone negatively because they're attracted to someone of the same sex to be a form of homophobia.
It is not considered sinful to have Homosexual thoughts or desires, acting on them is however, but only for those of the Catholic Faith and that is only if you interpret the Pauline Epistles in that way, there is a valid interpretation that counters much of the Catholic doctrine on this subject.......however that doesn't mean that the Catholic Church promotes homophobia against those that do, quite the contrary. The Catechism of the Catholic Church clearly states that "
[Homosexuals] must be accepted with respect, compassion and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfil God's Will in their lives, and if they are Christians, to unite to the sacrifice of the Lords Cross the difficulties they may encounter form their condition."
Now, I personally do not like the terminology used such as
Condition as that implies they are suffering from an abnormality of some kind...however I am not a Catholic or religious so that is to be expected....however I think you would be hard pressed to say that the Catechism supports Homophobia. The Catechism also tells
"States to do away with criminalising Homosexuality, is opposed to violations of their human rights, opposes all forms of violence against homosexuals and believes prejudice should be confronted at all levels, particularly State Level."
John Paul II said that the Church recognises that they do not choose their Homosexual Condition and for most of them it is a trial....again I am not entirely happy with the wording, but the idea is that homophobia is frowned upon, if not expressly forbidden by the Catholic Church.
The current Pope said in 1985;
"It is deplorable that homosexual persons have been and are the object of violent malice in speech or in action. Such treatment deserves condemnation from the Church's pastors wherever it occurs." (although he also reiterated that the act itself is
"it is a more or less strong tendency ordered toward an intrinsic moral evil" so we have to balance the positive with the negative.
Now you could argue that all is contrary to their stance against Gay marriage, however form their persepective they are not arguing against gay marriage, but against redefining marriage as it is currently as a
Union between a Man and a Woman....
Personally I think that the Church is struggling with it's own doctrine and how that informs modern Catholics, like most organisations of it's size and scale it is often somewhat anachronistic when compared to the actual beliefs and positions of it's congregation, the apparent contradiction of Cardinal Ratzinger illustrates my opinion...but it is changing, slowly, but changing nonetheless. There are Catholic Theologians and Cardinals that support the full acceptance and reinterpretation of Scripture and therefore doctrine, and others who oppose that. Time will tell.
I think that's a sad state of affairs in China and I wouldn't disagree that politics can promote such horrible things as homophobia as well, but I don't think that excuses religion to do it as well.
I am not excusing religion, what I am doing is giving an example that it is not only religion that can be used to justify prejudice or the promotion of such and therefore the call to remove Religion or a world without religion would not, in all likelihood, be any better....it could, given the inherent strictures in most religions be somewhat worse, because religion, as well as being a tool to justify prejudice, it can be a tool to be used to combat prejudice.
Edit: You said that China traditionally had an acceptance of homosexuality, and I don't know anything about it but you then go on to suggest that State enforcement (an outside influence from the individual) has resulted in homophobia manifesting itself? Or am I reading that wrong?
Manifesting itself in the State paradigm....so it made prejudice against homosexuality acceptable, those that would have been open to prejudice have an outlet for it, those that are not open to such prejudices would not.....the example was more to do with the differences or similarities between homophobia in Religion and Homophobia in a non-religious environment to illustrate that people will justify and manifest their prejudices in other ways if religion is not open to them. It is a factor of Human Nature, not of belief in God. Over time (since the death of Mao) the Chinese State have softened their approach toward homosexuality on a State Level. So while Communism (like Religion) can be used as a tool to promote prejudice, it is dependent on the interpretation of the individual as to how that manifests itself.....unfortunately with examples such as China and Stalinist Russia, the absolutism inherent in the power-base meant that the will and prejudices of the few manifested as widespread oppression and persecution of homosexuals...religion generally avoids this as it has doctrine that supersedes the authority of the individual.
, I'm in no way suggesting that religion incites murder/racism as that's up to interpretation but what I am suggesting is not up to interpretation is that religion encourages holding a faith position, even if (not definitely so) contrary to logic or evidence, and that is harmful. I'm not blaming specific religions either. I'm saying that general theistic belief is harmful; not deism, pantheism or a sense of the transcendant.
I'm aware that's not exactly what you're saying but you'll get a taste of what it's like for me reading your replies! 
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