Yes, feminism, however you define it, should not be forced into the setting or onto the character. It should be reasoned out. Even if its done on unreasonable levels (all men are swine), the character will have reasons behind it. They can be anything. Others have covered that bit better than us earlier here. no need for a recap on what might explain it.

In one RP we're in (The lower one in sig), we've built up a society in one Empire where genders are equal, while most others have a more traditional medieval setup. That society is constructed differently to make it work in a world so thoroughly different elsewhere. One easy way to implement such into a society is to borrow traits from other stories or the real world. You don't need to invent every bit of the society yourself. If there's a method/reason you particularly like in some book, just copy that and blend in a few of your own ideas. Saves a lot on time for making the design work. We've personally borrowed elements from at least three sources just for one group of people in that Empire. Other groups in the same empire don't share all those sources. Its more work that way, but the society ends up being more dynamic and considerably less "flat".

As for how we generally bring such elements into any RP: We do it by examples, by reasoned logic. If someone grew up with an abusive father, she might not be particularly fond of men. Or she might be abusive herself. If a female character got orphaned young, she is probably pretty strong personality-wise to have survived to adulthood. If a male character was raised solely by women, he would probably treat women much better than the average person. Or, if they couldn't understand him fully, he might end up disliking them. There are so many complexities to characters that nothing is pre-determined from a single set of traits, no matter what gender they have.