In Pattaya, people will often think it's a bar girl with her customer going to a short-timer hotel.
My wife had in the past ridden me home when I got too many beers but honestly, I was very scared feeling that she had problems controlling the scooter with a heavy man on the back.
Most bikers don't like to sit on the back, they want to be in control themselves and I am no different so I always ride the scooter (unless drunk off-course) and my wife is fine with that.
Then we have the family aspect: In Thailand the man is considered the head of the family so it's sort of an unwritten law that the man ride the bike and the rest of the family are passengers.
OP don't worry too much about if your GF is a good rider.