my own experiences with this? ... it's rough. some things I think are necessary for it to work. the 'bi guy' needs to be 'out' and comfortable with it, otherwise you'll always be a hidden side-thing. and that's not cool. you spend time together and he still can't introduce you to his friends, call you a "bf" or whatever? all. bad. signs. I think also some ground rules need to be talked about ... he wants to see a girl at the same time? you'll need to know what you're comfortable with -- maybe "ok, sure, but then you can't prioritize her over us... if we have plans, are out doing something, etc., then she can't call and have a "crisis" and demand attention, or show up and you disappear with her leaving me alone before our date is over", stuff like that. whatever ground rules you think matter, you gotta talk through and understand were you both are and be cool with that, or again, it's just not going to work out well. can it work? yeah... if both parties are mature and honest and work out the ground rules if not up-front, as things come up.