Conventional academic wisdom to the contrary notwithstanding, morality is emphatically not a social construct. To be sure, cultural differences exist, but this fact does not entail a lack of objective morality any more than the existence of different physical theories implies the non-existence of the physical world.
Moral relativism, or the thesis that morality is a social construct, is a tool for those who would establish a Dictatorship of Relativism, wherein only those strong enough to impose their will are right. It is the preferred doctrine of tyrants and evil people.
The Lord has engraved His law upon the hearts of all men. Those who detain the truth of the Lord are inexcusable (Romans 1:8).
To paraphrase Chesterton: when I was younger, I thought there were lots of "isms," but now I realize there are only two things: the Church and her enemies.