You trick her

"Hm.. I have a plan" you tell Supergirl and whisper what to do

A few minutes later Supergirl flies both of you over to Starfire.

"Starfire! I need your help! Using my x-ray vision I can tell that this man is dying but the only way to save him is.. well... " she starts

"What?! Hello, what is it to save him?" Starfire inquires

"He needs to cum, but I just can't seem to do it.. please help" Supergirl carries on

"Uh.. that is a thing here on Earth? Needing to cum or dying?" Starfire asks

"Yep, look at the pain he must be in" Supergirl says as she pulls free your massive erection and swings it in Starfire's face.

"Oh yes! There must be a lot of the pain for this human!" Starfire exclaims before taking your entire shaft down to your balls.

Hm, that was easier that you thought... now how is she gonna make me cum

"Hm... how shall I make this massive man cum... I think I shall..." Starfire starts to say