If this is actually yuri and not futa, how exactly are they supposed to be having sex in that position?

strap-ons?

pixiv title says something about toys.

Who cares? it could be one of three things. A futa, Toy usage (double-ended dildo..not EVERYTHING has to be a strap-on), or simply the girl from behind pussy humping her. Some girls DO that ya know (eye rolls) so who cares if it isn't perfectly Tagged? just enjoy the damn pic. Sheesh.