To the question of does God exist, the answer is whether Intelligence created Life

[panpan]

Ethics is not subjective, nor is it merely a social construct. It is the logically inevitable necessity that arises from acquired consciousness.

If ethics is not subjective then why do different societies have different ethics and why does ethics change over time? For example, in some societies gay marriage is immoral while in others it's moral? Or why was slavery in one society deemed as moral but then seen as immoral if ethics is objective?

Ethics is based on autonomy and non-harm of the autonomy of others. Even though every creature is forced to harm in order to survive, without wanting to harm itself, intelligent beings such as humans can limit this behavior based on morality, minimizing the harm they will cause to others and to the whole. 

Your observation is not wrong because ethics is usually determined by the personal interests of individuals and groups.