If I understand correctly, proper teshuva for sins between man and G-d removes punishment (Hilchot Teshuvah 2:9):
Neither repentance nor the Day of Atonement atone for any save for sins committed between man and God, for instance, one who ate forbidden food, or had forbidden coition and the like;
Further, we read (Hilchot Teshuvah 6:2):
That is saying concerning a sinner as long as he did not repent; but if he did repent, repentance to him is then a coat of armor against punishment. And, to the same extent that man sins consciously and of his own free will, he should turn to repentance consciously and of his own free will.
We also see in Hilchot Teshuvah 2:9 that adultery seems to be a sin between man and G-d according to Rambam.
Therefore, proper teshuva should remove punishment for this sin, shouldn't it?
However, in Mishlei 6:29 we read:
כֵּ֗ן הַ֭בָּא אֶל־אֵ֣שֶׁת רֵעֵ֑הוּ לֹ֥א יִ֝נָּקֶ֗ה כָּֽל־הַנֹּגֵ֥עַ בָּֽהּ׃
It is the same with one who sleeps with his fellow’s wife; None who touches her will go unpunished.
Therefore, does atonement for adultery always require punishment or only if one didn't do proper teshuva?