The girlfriend material is the sort of girl a guy would want to have fun with. To date, to be with, but not to have a serious relationship that would lead to  marriage. They are the women these guys would seek out when they plan to enjoy their bachelorhood and are not yet ready for marriage.

Such women are fun to be with. They tend to be glamorous and flirtatious, the life of the party. Probably the sex pot, hopefully commitment phobic too.

An old friend once told me he would never marry his first girlfriend. He'll have a series of girlfriends, date different girls before settling down with the final one who suits him best.

I know of 2 sisters, both pleasant looking an attractive, both nice girls. One of them is regarded among guys as wife material. She is warm, caring, rather motherly, sensible sort of girl. Not many men chasing her, but those who do are serious about marriage. Wife Material. The other is very outgoing, fun loving, part of the in group with a string of men after her. Not all the men after her plan to marry her. She's fun to be with. Girlfriend material. Both eventually married. 

Being branded girlfriend material doesn't necessarily mean that a girl would stay single and no men would marry her. She just has to look carefully at the men. Many would be out for fun. She has to be able to spot who are the guys who are serious about her.

Don't give guys the wrong impression. If you are looking for a serious relationship, don't flirt with other guys in the presence of your boyfriend. 

Watch what you wear. The clothes you wear send signals to guys as to the type of girl you are. Men do judge women by their appearances. Dress in ultra tight, micromini dresses that bare your shoulders all the time, paired with man eating high heel shoes and heavy makeup, and you'll come across as a loose girl. Someone to sleep around with and definitely not a candidate for marriage.

Dress sensibly, dress sexy occasionally if you like, dress practical most of the time, and you should not come across as a slut.