When your girlfriend wants to stay at a guy’s house and you have a problem with that, use diplomacy to make your point. If you think she’s cheating and she’s not, for example, you could offend her and make her very angry. Clearly explain your reasoning and listen to her response. A compromise might be necessary for you to get her to agree to your request, or you might have to trust her.

Carefully Manage Emotions

Effective communication includes giving information and coping with the emotion behind the information, according to therapists Jeanne and Robert Segal and writer Lawrence Robinson in “Effective Communication -- Improving Communication Skills in Business and Relationships” for HelpGuide.org. Identify the emotions that you feel when she spends time at a guy’s house, such as jealousy, pain, frustration, fear or betrayal. Explain how you feel and why it bothers you. If you think she isn’t safe there or that she might be inclined to cheat, state your concerns without accusing her of any wrongdoing. Stay calm during the conversation by breathing deeply, tightening and relaxing muscles or pacing.

Her Reasonings

Ask her why she wants to stay at the guy’s house. If you feel that her actions could be cheating or lead her to cheat, she might not agree. People can have different definitions of what constitutes cheating, according to a 2013 study about perceptions of cheating published in the “Evolutionary Psychology” journal. Get her perspective on what behaviors are cheating and try to synchronize them with yours. Alternatively, if you worry about her safety and she feels perfectly safe there, listen to why she feels safe and try to see things from her perspective.

Set the Stage

Go into the conversation with a clear idea of what you want to say, according to the Family Education Network in “10 Steps to Clear Communication.” Have the conversation when you two can talk calmly and without interruption. Listen and speak, repeating her comments back to her to clarify her message and asking her to do the same. Part of the goal is for both of you to feel heard and understood, especially if there is a disagreement about whether she will comply or not.

The Art of Compromise

If you two do not see eye to eye on this matter, you might have to compromise, the Segals and Robinson suggest. Perhaps she will agree to only stay there when she and the guy aren’t alone or during certain activities, such as a group study session or a chaperoned party. When the guy is someone you don’t know, perhaps you will feel more comfortable after you have met him and had a chance to see how your girl and the guy act when together. Look for win-win solutions, such as when she feels you trust her but agrees that you have reasons not to trust the guy, so she stays only if someone else is present.