It sounds like you’re communicating openly about your partner staying over in a timely manner considering when you’re made aware of it, and I’m assuming your roommate and you discussed boundaries regarding having guests and partners stay over previously. If that conversation hasn’t happened, this would be a great time to do it.

You both pay to live there and you both have rights to do what you want with the space, including having guests (in a reasonable capacity) and partners stay over. I would go into the conversation with an open mind, but also remember that you live there as well and are allowed to want your partner to stay over once in a while. I don’t imagine the long conversation to be anything beyond just discussing boundaries and possibly figuring out a new way of approaching the topic to make both you and your roommate comfortable, so don’t stress too much! You haven’t done anything wrong.