That's true. Arguments for sexual license are not usually done. I suspect that they haven't been engaged in seriously since the Enlightenment given the degree to which license is the default position rhetorically. Usually now, the general argumentative tactic is to presume that sexuality is not morally special and then to presume a utilitarian framework. But, most people who hold a left liberal view of sexuality don't start that deep. They think that homosexuality and other forms of sexual behavior simply should be amongst the other kinds of activities that are already permitted.