Do you think it is wrong for a Christian to be a male model? If so, do you think it's wrong for a male model to pose shirtless?
It depends on what the modeling is for. To be in advertisements for many items would be fine. But to model, say for a beer commercial, would not be right since it would be promoting drinking. To pose for sensual pictures would not be right since Christians are not to lust or encourage lust. "Therefore, since Christ suffered for us in the flesh, arm yourselves also with the same mind, for he who has suffered in the flesh has ceased from sin, that he no longer should live the rest of his time in the flesh for the lusts of men, but for the will of God. For we have spent enough of our past lifetime in doing the will of the Gentiles -- when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries" (I Peter 4:1-3).
Generally shirtless male pictures are aimed at stirring sensual feelings in the viewer (not always, but often). This would not be appropriate for a Christian to be involved in.