More than 50 constitutional law professors urged President Obama on Monday not to include a religious exemption in an expected order barring federal contractors from discriminating on the basis of sexual orientation and gender identity. Conservative religious leaders want the rule, still being drafted, to exempt religious groups that object to homosexual behavior. But legal experts, organized by Katherine M. Franke of the Columbia University Law School, said in an open letter that church groups are already allowed to favor members of their faith in hiring, and that a new exception would allow employers to discriminate using taxpayer dollars and enshrine a second-class legal status for gay, lesbian and transgender workers.