"A household of three" (a literal translation of the above) commonly appears in fiction, but many say it should not enter into reality. Whether it is wrong or not is far clearer when both partners are of the same gender, but what if they are not?
Importantly, why is it wrong when they are both of the same gender? They conflict with one another. Every bisexual I have ever spoken to about this agrees the same thing: men and women are different. It's something that is, in my experience, very hard for monosexuals to grasp, simply as from their experience anybody they are attracted to conflicts with anyone else they are attracted to.
Therefore, it can be said that boyfriends and girlfriends conflict in a single manner: attention (e.g., time, money). These can be dealt with in most cases by simply giving both partners equal attention, ideally spending twice as much time and money as would be done with a single partner.
If done properly, it is possible to have two partners, one of each sex, without either ever infringing on the other.