That's just your opinion.
No
a priori proof has ever been offered for the existence of a single, objective, standard of morality.
So saying that this is "just" my opinion is like saying that a disbelief in unicorns is "just" an opinion.
I'm perfectly willing to believe in objective morality if someone can prove to me that it exists.
Absent that, belief in objective morality is like belief in the Lock Ness Monster, faires, or dragons.
I'm willing to bet that your belief in objective morality is somehow tied to your belief in some religious doctrine or theology (or, much less likely, to Kant's categorical imperative, which though in itself objective is really subjective as it leaves the determination of what rules should be objectively held up to each individual).
Anyhow, you believe that some standard of morality is objective because of an appeal to what you've been told about God.
Thing is, not everyone believes in the same God or religion.
Consequently, not everyone holds the same moral standard, and it's possible for people to hold two very different moral standards and for both to claim that their standard is objective.
If two people hold differing moral standards and both claim that their standard, based on their understanding of their God/religion is objective, how do we decide who is right?