If someone is dating a girl who also has a boyfriend, should the 2 girls have to remain casual with each other or could they also love each other? The girl I'm into has had a relationship with a guy and a girl who had a serious girlfriend, but I wouldn't mind her dating him alongside dating me... It's just that me and the girl have really good chemistry and it would probably get quite serious even though I know that she might want to stay with her boyfriend... This would seem ok to me but what about him? Would he be ok with it, or would he think that me and her should only be friends with benefits...?
Each relationship is different since not every type of relationship works for all people. Perhaps you three need to sit down and discuss what you want out of the relationship and if everyone is okay with her being serious with both you and this guy. I don't think there is a set rule here at all, but that it is up to those involved to set the rules.
This isn't a bisexual question, this is a consensual non-monogamy question, and the answer to most of those questions is healthy communication.
This sounds more poly than bi to be honest because bisexuals can be with anyone and in monogamous relationships with lesbians , straight men , and other bisexuals .
There aren't "bisexual relationship rules." It's determined by all the people who agree to an open relationship and the terms on which they agree. This is not a sexual orientation thing, this is a polyamory thing.