Girlfriend is "neurospicy" and her current special interest is having sex with a guy with a bigger penis than mine

I'm away from campus and working as a research assistant at a federal lab for the summer. My girlfriend is taking summer classes and is working part time as a barista. We met in Logic Design of Digital Systems. She has diagnosed anxiety and mild aspergers, which we have been working through together. She means a tremendous amount to me, and has helped me through some difficult times during my sophmore year as well.

Basically, she has met this guy in the coffee shop and is deeply sexually interested in him, and suspects based off his physical characteristics that he is probably better endowed than me. We do NOT have an open relationship but I know she has issues with impulse control. Should I let her have sex with this guy?