Societal morals are in many cases wrong at times due to the fallibility of man. However, in the absence of regular communication from the divine to clear up misunderstandings the adaptability of societal morals over time becomes far superior to those of a divine nature. Man learned that slavery was wrong and man fought against the practice and freed the slaves in many areas of the world. God did not come down and perform miracles or command we stop using slaves, we did. Certain laws are easy to see through societal contract. laws against murder, theft, assault, and other laws protecting the sovereignity of life, physical safety, and of property make sense to religious and non-religious alike. None of us want to be assaulted, killed, or have our stuff taken so we agree that we will not do those things, and that the law will be present to punish those who do so and remove them from society for not abiding by such social contracts. I can even see the argument against abortion by the idealists who truly believe in the sanctity of life under those guidelines, though they often resort to a piss poor religious argument which has no basis in truth because they cannot prove god exists or that their religion is the religion of said overlord of the universe.
Morality becomes subjective when you start dealing with laws that are not gotten through social contract. gay marriage is one of those. there has been no proof at all, and even proof to the contrary, that gay marriage is harmful to anyone but those who happen to willingly participate in it. Unlike something like assault your life is not directly effected by gay marriage. Yet it is claimed as being something moral based upon divinity which has no basis in truth. That is a moral value relative to the person making it. Those are moral choices a person makes on an individual basis as they are the only people harmed or benefitting from those choices.
This is why morals are relative. We see that at different points in time and different places even society declares different values for laws. It was legal to own slaves. it is legal to assault some people, or to kill at times. Those positions are relative to the situation and environment you happen to be in. Even religious moral values are relative to the religion you practice or do not practice. It is moral for a muslim woman to wear a hijab, but a christian woman is not concerned with that morality or even finds it to be moral at all. Until your prove god, and you prove a specific god and set of rules your claims that morals are divinely sanctified are just spouting at best. if it is truly moral like no murder you can make an argument without the use of god or divine inspiration which you cannot prove. If you need to use god to prove your moral stance remove the relativity in your stance and prove god and what it wants to us.