Poised.

This little tart thinks that this clumsy advance can win you over? Whatever her game is, you aren't having it. You're not going to let her get a rise out of you.
"Either come inside or get out, Tara, but I suggest you do the second one." You stop jerking off, and rinse off completely, turning off the shower. You step out of the shower and towel off, but you don't rush to cover yourself or panic. You just go about your business as if she isn't there. "You should probably cut that out. If anybody walks by, you're going to be pretty embarrassed."
You get the satisfied sight, out of the corner of your eye, of Tara's jaw dropping. She's momentarily dumbfounded at your reaction, or rather, your lack of reaction. She recovers quickly, though, gliding into the bathroom to press her tits against your arm as you run the electric razor.
"Wow Mr. Sinclair. You're so... confident. I like that. Most guys would get embarrassed or angry-"
"You invade a lot of boys' bathrooms uninvited, huh?" You're not playing her game.
She recoils for a second, as if you had slapped her. In the mirror, you can see her trying to hide her incredulity. In a way, you almost pity her. She's probably never had any trouble manipulating boys with her looks and charisma. She doesn't really know what to do, now.
"You have a nice cock, James. I saw it. Much nicer than Jared's. Maybe I should trade up to the full-size package." Ugh... you wince at the terrrible wordplay. She snakes an arm around yours, again rubbing her tits against your bicep. "I bet I can show you things that she can't..."