The argument is that if all morality is subjective it would be relative. It would not have a fixed, universal measure. Simple as that.
Please demonstrate this fixed, universal measure of morality.
How could I ever do that to your satisfaction? I learned a long time ago that you can't convince someone against their will.
I can and have given you reasonable and logical arguments.
I have explained the idea of a Necessary Being. I have explained that the biblical God is revealed as omniscient, unchanging, omnipresent, omnibenevolent, and eternal. Those qualities or attributes are what are necessary for a fixed universal measure.
And if you're going to insist on "love" as your primary moral AXIOM, there are an awful lot of rules you're going to have to change.
Such as?
Not to mention that every single element of your definition of "love" is pure Qualia.
The biblical God as a necessary being would be the standard. Only if/because He has revealed Himself and given us an example could/do we have a reference point. His nature reveals the extent of His love and justice.
Love is patient - God is patient with us. We are given many chances.
Love is kind - it keeps no record of wrongs for those in Christ Jesus for He has met the standard God requires.
Love is not arrogant or boastful.
Love does not take into account a wrong suffered - God forgive in Christ Jesus but since He is just (a just judge cannot overlook a wrong. He would not be a good judge to wink at evil) wrongful actions are also addressed, either in our own merit or in the merit of Another.
Love does not rejoice in unrighteousness - hence, we all come before the judgment of God for our actions and life choices. Our wrongful actions are either paid for by us or by Another. Thus, justice is met.
Love rejoices with the truth - Jesus Christ is the
truth. Finding truth sets you free from error. Error cloaks you in falsity. Jesus said that a human being builds their life on one of two
foundations. Your starting presuppositions of either God or chance happenstance are important to how you build your worldview in how it makes sense of existence or is unable to do so.
Love bears all things, believes all things, hopes all things, endures all things - with God's love I can endure life's hardships. I know they are only temporary.
Love protects, and it
seeks the best - Jesus protects me from my own self. He guides me in times of trouble.
Love is just - How will God not do good? He allows us for a time to do our own thing but eventually, we are accountable. Hitler does not get a free licence in which all the injustice he is responsible for is not punished and answered.