ὦ ἄνθρωπε

Anyone who has engaged the issue of sexuality and the Bible has at some point contended with Romans 1:26-27: “For this reason God gave them up to degrading passions. Their women exchanged natural intercourse for unnatural, and in the same way also the men, giving up natural intercourse with women, were consumed with passion for one another. Men committed shameless acts with men and received in their own persons the due penalty for their error.” (NRSV)
...
What’s even more striking, notes Porter, is what comes next: an abrupt change to the second person in Romans 2:1:
“Therefore you have no excuse, whoever you are, when you judge others; for in passing judgment on another you condemn yourself, because you, the judge, are doing the very same things.”
Here, then, is the vocative in the Greek, “Oh man,” a grammatical case used for direct address: ὦ ἄνθρωπε. And this takes us to the question I have posed to those who repeat 1:26-27 in condemnation. Who’s the ἄνθρωπος that Paul’s addressing here?