XX males have no Y chromosomes, yet they are males.
For ~90% of XX males: one of their X-chromosomes functions as a Y due to SRY-insertion (SRY is the sex-determining region of the Y-chromosome).
The other ~10% of XX males are male despite not having SRY on an X (they are “negative for SRY gene” or “SRY-negative”).
I don’t think anyone knows why these 10% are male: even today the genetics of sex-determination are not completely understood.
How is one biologically male without Y chromosomes? I remember from science class that that is essentially what makes us male.