Its all about dominence. The one doing the humping wants to be the alpha dog. Mine do it still after being fixed. Good Luck!!
And consider moving to California, where your dog can marry her doggie girlfriend legally. But she could only be bi-curious.
Seriously, its a dominance thing, mostly.
Consider neutering them, in all seriousness. 2 intact females eventually will spell trouble for your household.
A dog will hump anything! A leg, stuffed toy, each other, etc.
If she is a lesbian, do as any mother would do, and ACCEPT her for who and what she is. LOL