I mean technically what Rachel does is purely functional: the collection of and absorption of mana. Though it may be functionally sexual, apparently she has no sexual nor romantic intent with the act. So, Xiya being the one who wishes to and tries to engage in romantic and sexual action would technically be the top.
Now if Rachel gets her bloody act together and stops with the bullheaded denseness maybe that would change! I can understand Xiya’s frustration here. Rachel’s “well technically we are bonded for ever and ever and technically we do sexual stuff, but it is purely a utilitarian relationship which I do not want to label as anything other than that because it would be an ethical minefield.” act is getting really old! 😤