A boyfriend who is just there to satisfy the need for having a boyfriend.
i.e. someone who is fulfilling short-term needs in your life, but for whom you don't see a long-term purpose.
Eh, he's just a placeholder boyfriend in my life. I hate sleeping alone.
I don't get fazed when I'm into a girl who has a boyfriend. I always just assume that he's a placeholder boyfriend, and that she'll dump him once she finds someone real.