Wow, it sure sounds to someone who doesn't know anything else about your guy or your relationship in general that he thought he was mono, has recently discovered he might not be, and thinks it will be a little less like cheating if he brings her into your bed. Although I realize this might be very common for people just starting on the path of non-monogamy, to me it seems disrespectful as hell.
Most relationships end up blurring the lines between me and you a bit. But this is his discovery, his project, and that doesn't mean you have to get along the bandwagon with him.
Me: bi female in my twenties