You rightly note there is no monopoly, gay or straight, on abuse. Your argument suggests that the popular perception that being being gay is a sign of poor character, (which carries the corollary, that being straight is a sign, if not proof, of good character) is an adequate justification for policy. That seems naive. Seminaries struggle and too often fail to tell sound seminarians from the unsound, whatever their sexual orientation.  This move does not suggest any improvement in that process of discernment, and may make the job harder. 

The Church has always had effective means to deal with problematic members of the clergy. The aspects of the sex abuse scandals most damaging to Her reputation have to do with her many failures to employ those means. I worry that the Church, by denying priesthood to homosexuals based on their sexuality alone, is attempting to shift attention from Her own failings. Which would be yet another failure.