Why does the boyfriend always sit between his girlfriend in the middle?

Every time I am riding the bus or in public, a guy and his girlfriend will always be sitting by me AND I AM ALWAYS THE ONE SITTING BY THE GUY!! The guy seems to be the one who wants to sit in the middle and not allow his girlfriend to sit by strangers! Why is that?! Most cases, if a guy and his girlfriend will ride the bus, his girlfriend will always sit on the end of the side seats and the guy will be the one squishing me and sitting by me for some weird reason! Why is that?! It drives me nuts!! He must be gay or something or don’t want his girlfriend feeling uncomfortable with personal space!