Why does girls always in the disadvantage?

One of my friend has been troubled with relationships lately. When i talk to her, i get into deep thinkings. I don understand why is the girls always in trouble with their boyfriend. But when i turn and ask the guy, the guy doesnt seems to care much. Why are girls always at the weaker side. I seldom see strong, brave girls. Even one of my friend, the strongest girl i've seen, gone through millions tons of obstacles, have her weak side too.. Does everyone have their real weak side? Why didnt i see the guys having that too.. I just very ''bu fu qi'', why does girls always kanna penalised, even in the olden days, just because girls are weaker physically? In everyone's mind, guys are always protecting the girls, even to me. But why does everyone have a concept that guys are the strongest? From the day i ''dong shi'', this mindset has been fixed. Izit that because all the while man just pass down this concept? In a relationship, why does girls always got hurt, got bully? Why doesnt the guy do their part, as what a guy should do in a relationship, as how much effort they should put in. Wy does there is so little trust in every relationship that i know. And why does so many many questions came out suddenly..