[MermaidMassacre]

You two do look alike =P
People are just still not use to the idea that girls and guys can be close friends without being more. So just ignore the rumors and tell them how it is.
His girlfriend should understand if he explains it to her.
It's a simple mistake.

My friend was really close with her cousin and they went to the same school. No one knew they were cousins so they all assumed they were dating always hanging out with each other.
So kinda like your situation...only they were actually cousins. =P