Just curious what you think - if opinions are split within a society, then whose opinions should inform the law?
You are oversimplifying the issue. Depends on the issue and if decision made harms society or benefits society or government and a whole load of other factors. Are you trying to suggest that government should form a law based on the majority's opinion ALL THE TIME not taking into consideration other factors?
Since we are talking about homosexuality, I'm going to give my OPINION in relation to that. I would prefer to keep it in Singapore context since I am, after all, a Singaporean and also because different countries have different dynamics and it will not be possible to discuss all the different scenarios in this space...
As for Singapore, I THINK that the government should allow same sex marriage (ssm). Not now, but when there ARE enough SIGNIFICANT minorities who are NOT against ssm. Singapore society is evolving. Sex-changed marriage is allowed here. Won't be long before they allow homosexual marriage too. My opinion

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The majority, those with God's backing, or those who agree with you?
Begging the question. You are starting out with premises not yet proven in the first place. As to date, there is no proof that there is god. And Singapore IS A SECULAR country. Singapore DOES NOT MAKE rules based on God or anyone's arguments based on God only.
I'm also curious. I would like to turn the question to you. In a dominantly pro-ssm country, whose opinions do you think should inform the law? The majority pro-ssm people or the minority who feel that their unproven god and religion should dictate the law?