Which part is disturbing you? The “sometimes” or the “girlfriend”?
If it’s an open relationship – ie, you’re not exclusive, then presumably you’d be okay with her having a “sometimes boyfriend”. So if it’s the girlfriend part that’s bothering you, sorry, she’s not going to suddenly become not-bisexual just for you. If you can’t accept that, get out now.
If it’s a closed relationship – ie, you are exclusive, then she obviously doesn’t feel that way, as she has something else on the side. If you can’t deal with that, get out now.