how do i ask my bf if he's ok with an "open" relationship?

If he is your bf, then it is not an "open" relationship

That's a toughy. I would suggest that you ask him straight out. I had a long relationship with a woman, and we lived together, where it was open. It really depends on how he feels about you and how you feel about him. If you're talking sex he may be open to that. If not, probably not. It's a difficult question to answer when people don't know you. Some, but not many men, may go for it but I know of very few who would like it.

If you mean an easy-going "****-buddy" type thing most men would like that. Horses for courses. These are things you can only really work out for yourself, and become a better person for it.

Maybe you could bring up a hypothetical situation, or say something like, "Hey, did I tell you about so and so? They have an open relationship, and they say it works great for them. What do you think about that?" and just see where the conversation goes from there. Who knows?

Until you're married it is an open relationship.

The problem is the deception, not the infidelity.

I would just ask, and be prepared to tell him who you want to f***.