I don’t think it’s from a place of not taking f/f seriously. It could be, sure, but in any real life case of this it’s been from a sense of entitlement around expressing their sexuality. “It’s not cheating because I’m allowed to explore my sexuality!!!” This is an actual quote I’ve heard out in the world, btw. It’s a tool used to avoid accountability, a common technique for abusive partners to use. Which can happen in any type of relationship and we should never try to assign it to any one type
Like I’ve fr heard straight up DARBO techniques of “you’re just homophobic/biphobic for telling ME I’m not allowed to express my sexuality while in a relationship!” Or “it’s just sex! It’s not like I’m dating them too!” (common cheater excuse regardless of sexuality/gender/relationship type) but never once heard of them saying or using an excuse leading to f/f relationships not being considered serious