I think that at one time Bellatrix loved Rodolphus; maybe before they both became death eaters and involved in their work. She is not the wife of the Dark Lord, but I am sure she would like to be because it would boost her ego as to thinking she really is the most loyal servant! Their true relationship would be that Bella fantasizes about the Dark Lord, and would want to be in a relationship with him - it is more loyalty, admiration, and envy though. He, however, feels nothing for her in the romantic sort of way, because he has no love left to give. I think it was DD or one of the marauders who said that Voldemort does not know how to love (or perhaps i read it in a fanfic... not sure). So yes, Bella can lust after Voldy all she wants, but it won't do any good if he doesn't know how to return that love.