It's understandable that you're feeling uneasy about your girlfriend's suggestion, even if it was made in jest. Here’s how you can approach this topic with her in a constructive and respectful manner:

1. Choose the Right Time and Place

Find a calm and private moment to discuss your feelings. Avoid bringing it up during a casual conversation or when either of you is distracted. A quiet evening at home or a relaxed weekend could be ideal.

2. Use "I" Statements

When you express your feelings, use "I" statements to avoid sounding accusatory. This will help her understand that you are sharing your feelings rather than blaming her for suggesting it. For example:

• "I feel uncomfortable at the thought of having a male friend living with us."
• "I need to be able to feel at home and have my space without feeling like I’m sharing it with someone else."

3. Acknowledge Her Friendship

Make it clear that you respect her friendships and understand that she values them. You might say:

• "I understand that you have a close friendship with him, and I think that's great."

4. Express Your Concerns

Clearly communicate your concerns about how this arrangement could affect your relationship. Focus on your feelings rather than making it about her actions. You could say:

• "I worry that having him around all the time might make me feel less secure in our relationship."
• "I also feel that I won’t have enough alone time with you, and that’s important to me."

5. Suggest Alternatives

If she's looking for companionship or support, suggest alternatives that don't involve living together. For example:

• "What if we invite him over for dinner or game nights instead?"
• "I think it would be better if we maintained our own space while still nurturing your friendship."

6. Be Open to Her Perspective

Encourage her to share her thoughts and feelings about the situation. Listen actively and validate her feelings, even if you disagree. You might say:

• "I want to understand your perspective on this, so please share what you think."

7. Reaffirm Your Commitment

End the conversation by reassuring her of your commitment to the relationship. Let her know that your feelings come from a place of wanting to maintain a strong and healthy partnership. For example:

• "I love living with you and want to make sure we both feel comfortable and happy in our home."

By approaching the conversation with empathy and clarity, you can express your feelings without coming across as controlling or unreasonable. It’s important to maintain open lines of communication in your relationship, and addressing this concern thoughtfully can strengthen your bond.