yes, because mine is possesive and most people usually are when it somes to relationships... as to where "just a friend" is exactly that. The sex differential is just how the German language is.. Most things must be either Male, female or neutral (hence Der, Die and Das.)
It would be like telling someone in english "She is my GF" or correcting a person saying "Oh no, she is just a friend."
A german teacher told me that is you want to express a friendship you should use: Sie ist eine Freundin von mir (She is my friend (girl)) and for a relationship deeper you should use: Sie ist meine Freundin (She is my girlfriend). This is only for clarifying in some cases, when it is necessary to be pretty sure about it.
Sometimes women refer to their female non-romantic friends as "girlfriends", but I haven't seen "boyfriends" used in this way (although maybe it is in some communities).
In German, I've also heard the same usage of meine Freundin for a woman to refer to her non-romantic friend where the context is clear.
There is, apparently, a bit of awkwardness for same-sex couples as there's sometimes no easy way for them to clarify that they are dating rather than 'just friends', as is the common assumption when you refer to mein(e) Freund(in) of the same gender as yourself.