You tell her she must be joking

"That's preposterous! Altough I must say you look spectacularly fine today, I'll only believe in superpowers when I see them."

"Watch this, then."

She gets up on her feet and stands with perfect posture. Then, without batting an eyelid, she departs from the jurisdiction of gravity and floats elegantly above the carpet. Her long hair flows around mid-air is if dancing upon the tensed surface of water.

You're sold. At least on this part of her claims.

"Wow that's so cool! Can you show me something else?"

"Sure."

For a moment, her figure turns into a mesh of parallel lines, before refocusing suddenly. She's turned around, and is now holding a three inch thick solid iron post.

"Where'd you get that? Don't tell me you can materialize things from thin air now?"

"No, silly! I got this from a construction site from downtown. I got super speed too, you know."

She proceeds to rip the thick rod into shreds with her bare hands. A detestable metallic screech convinces you that it's real, instead of rubber or liquorice. Judging by its behavior in her slender hands though, it might as well be. She finishes off her show of might by outright vaporizing the metal with laser vision.

On one hand you are slightly scared. Such blatant disregard for the laws of physics, the power to tear you apart piece by piece and then scorch you to a singe with a single look, all displayed with arrogant ease by none other than your girlfriend. On the other hand, this makes you incredibly aroused. Your hard-on rose to prominence from just seeing her fly.

"Holy... you're literally Super Girl, aren't you?"

"You mean you're ok with this?"

"More than okay. That was the hottest thing I've ever seen. I need action, and fast!"

"Phew! I was so worried you'd think of me as a freak. This is such a relief... oh."

You're halfway naked already.

"Please, do this for me! Even if this is a dream, I wanna fuck Super Girl just once in my life!"

"Hehee... Alright, but make no mistake, this isn't a dream..."

In another blur, her clothes vanish, and she flies down to almost touch your face.

"...I'm for reals."