Why is it OK, for males to compete against females?

Where? What country and sport?
There is no single rule across all sports regarding transgender female participation. Each sport sets its own guidelines based on its unique physical demands.
That being said, trans women that are allowed to compete against women are usually eligible once they have undergone hormone treatment. Meaning trans athletes have undergone suppression of testosterone therefore gender-affirming healthcare. They have no biological advantage.

It isn't lost on me that men that complain about this are usually quiet about women suffering sexual, physical and emotional abuse as on a much larger scale.
Unless a woman herself complains about a proven disadvantage in these sports, I don't think we need men with tunnel vision to do it for us. I say proven because the last time this happened, a biologically born woman's life was almost ruined (imane khelif)