She is testing the pack order. Has nothing to do with sexuality. My spayed female is the dominant dog in my house, and will often mount and hump my intact male, in play or in excitement. My male NEVER tries to hump her, as he defers to her pack position and seniority.
Since Xena is getting older (how old is she now?), pack dynamics may be changing, with her subtly trying to move up the pack. She is still somewhat unsure about this, that is why she is only doing it when he is lying down, in a submissive position to start with. If your Husky is not challenging her for pack leadership, let the dynamics be, BUT I myself would not allow Xena to annoy the older dog, since he clearly does not enjoy her lying on him. Just as I find the humping in play inappropriate, and tell my older dog to quit it when she does it, I would tell Xena to get OFF the Husky.