Tribadism? Wishful thinking I think.. looks more like Futa to me ;P (doesn't look like strapon.. can't be tribadism because of the angle and thrusting motion).
Look at her leg's though... they're crossing with the other girl's legs... so what if they thrust? lol they're really just wishing one of them had a penis.
Waaay late to the party here, but I'm going to have to agree that there's no reason this has to be futa. With their legs crossed as they are, it's almost certainly two sets of girl parts they are rubbing together.
The top woman is wearing teal/green. The woman on the bottom, "thrusting", has bare legs. Both teal/green legs are over the bare legs. On top, if you will. To not have your legs crossed with your partner's when preforming tribadism is quite uncomfortable and actually quite inefficient.
Plus, if you look closely at the bottom woman, her back is arching, which you can see by the fact that her ribcage and collarbones are rising. This means that her vagina is likely faces outward, rather than upwards. The teal/green woman is leaning forward, over the bottom woman, easily exposing her vagina to the hips of the bottom woman. Either their just bumping abdomen-against-vagina, or the bottom's a futa. And based on the bottom woman's expression, she's in too much ecstasy for the top to just be rubbing her cunt against a hipbone or something. Changing the tags to futa.
