Maybe at the time Leviticus was conjured up, the writers hadn't considered lesbianism. The role of women being what it was, it may have been a deep, dark , unmentionable secret known only to those who practised it.
I imagine the grounds for objection, if there were any, would be that 'man' can be taken to mean 'human', but whichever way you look at it, if the church wants to find something in the bible to back their case, they'll find it. We all know the bible doesn't mean what it says - unless, of course, the church wants it to.