I've sometimes been asked if I can prove the non-existence of god and when I agree that I can't I'm asked why I?m not an agnostic rather than an atheist (the same question applies to religious believers who agree that they can't prove that god does exist). The most written about argument against the existence of a theistic god (i.e. all powerful, all loving and normally some other things as well) is known as the problem of evil. If god is all powerful and all loving, why is there so much suffering in the world? Either god doesn't want to stop the suffering or he can't. In my experience, people are not bothered much by this argument when it's brought up in debate but it is often discussed publicly after major natural disasters like earthquakes or tsunamis. Attempts to evade the problem are known as theodicies, of which there are many. The most well know theodicy is the free will defence. It's more important that we have free will than we be happy all the time and having real free will means being able to choose genuinely nasty ways to occupy our time. As a consequence, the existence of serial killers is not incompatible with a god who loves us.
For the purpose of this post you'll have to take my word that the free will defence is generally regarded by both atheistic and religious philosophers as the best way out of the problem of evil. On this account, freedom to choose between good and bad things is more important that happiness or any other kind of material welfare. God doesn't just allow us to plan evil plans and then thwart us by giving us an epileptic fit or tipping off the police or the betrayed lover, he allows us to actually do bad things.
In a police state like that outlined in George Orwell's 1984, everything we did would be monitored by the authorities and if someone broke the rules he would quickly be whisked away. We are not free in a society like this and we wouldn't suddenly become free if Big Brother started strictly enforcing the (true) code of morality outlined in the Bible (or other religious text of your choice). The more closely the law enforces those moral rules the more it would seem easier if we were simply programmed to do the right thing in the first place (still allowing individuals "freedom" to like or dislike the things they did as they did them), which god consciously decided not to do.
So when religious people argue that maybe the state shouldn't do anything if someone is attracted to others of the same sex but that homosexual sex should be illegal, I think they are arguing against the kind of freedom that they rely on to justify why the world is often such an unpleasant place to be. If theists are statist then I think there is a significant tension in the belief system.
To be a good Christian, it is important that one chooses to follow Christian ethics. And while Christians may regret that society is not as Christian in character as they would like, they should try to persuade people, not force them to change their ways.
At least one person reading this will be thinking now that I'm arguing in favour of legalising murder and rape etc. I'm not. I'm not in favour of this because when I exercise my freedom to murder, I deprive somebody else of their freedom to make any choices at all. However if I decide to spend my money on heroin and destroy my life, that's an example of the exercising of freedom that doesn't deprive anybody else of theirs.