exactly. we don't know anything. and those are important details. if she is in an open relationship, good for her, but from how she worded all their interactions, it doesn't seem to be the case. we would need a clearer picture.MC knows she has a boyfriend. Maybe Bella's boyfriend knows and allows that Bella is screwing around. I don't see any cheating. I see a sex loving young women with options and freedom of choice without judging. If everybody is happy, I'm happy.
she seemed satisfied until she tried MC's dick. so it's more of a case of having found a better partner than her being frustrated. at least, from what we saw.If I remember correctly, she mentioned that the last time she had sex with MC was one week ago. I think her boyfriend doesn't satisfy her properly but for her this is no reason to break up with him because there might be other things besides sex she likes about him. She also knows that she can not have the MC alone for herself and someday the MC will have a GF and her fun with him will come to an end.
before the latest update, she didn't really know she couldn't have him for herself. just that she assumed they couldn't have a proper official relationship. now it's beginning to change so the boyfriend question might get a resolution in the ending next update.