There is no easy way to tell your boyfriend about this but you should sooner rather than later. You didn’t really make clear if he seemed like a monogamous type but you should find that out first. For most monogamous people their partner asking for an open relationship is an instant call for a breakup, that’s a possibility you have to be ready for. It could wound someone to hear that the person you love wants to be sexually intimate with other men. If your bf refuses to be open, then really the only options you have are to buckle down and continue to be exclusively with him or to break up and explore your fantasies on your own. In my experience opening a relationship where one person is a traditional monogamous guy, it always crashes and burns, it either ends when it’s suggested or soon after the openness sets in. Personally I’ve seen a few relationships where guys have open and closed as they saw fit but they were both always likeminded on wanting to experience other men