Romance can be lovely, but if you're the flatmate of someone who falls in love repeatedly - and feels free to share his or her space with each new partner - then no one would blame you for thinking it a bit inconvenient as well. But don't think about moving out (or kicking your flatmate out) quite yet. Instead, talk over your concerns with your flatmate and see if you can't come to some rational decisions regarding romantic partners.
Know Your Own Mind
Before you approach your flatmate to discuss his or her personal life, know your own mind regarding what is bothering you. Is it the number of romantic partners that turn up in your home? Is it that you don't like meeting new people over the breakfast table? Is it that a long-term partner routinely invades your personal space? Is it that you feel like you have a permanent new flatmate? Once you know what you are having trouble with you'll be more able to clearly address these issues with your flatmate.
Arm Yourself With Proof
Don't go overboard - you don't need to go around collecting forensic evidence about your flatmates' romantic partners but you may want to have a few facts on your side when you speak to your flatmate. For example, keeping track of how many times his or her partner has been over that week or month, how many different partners have greeted you in the morning or even if or how often your own guests have felt uncomfortable due to his or her romantic partners are all valid proof of your flatmates social habits. It may well be that your flatmate has never thought about the big picture and will be surprised to hear these statistics.
Be Ready to Compromise
It's highly unlikely that the minute you begin to speak your flatmate will renounce his or her amorous ways and give up bringing romantic partners home forever. It's far more likely that you'll both need to compromise a little on the subject. To make sure you get what you need, know exactly what you are hoping for before you even begin your conversation. If all you want is a night free from partners every now and then don't hesitate to say this. You never know, if you don't then your flatmate may end up feeling that you can't stand his or her partners and are asking for a complete ban.
Try to Make Friends
If your flatmate has become involved with just one partner it can help immeasurably if you try to make friends with this person. No one is asking you to become best friends, or even close friends, but being civil to each other and even being able to discuss work, school or common interests will likely make much of the time you spend together more comfortable for everyone. Though it may be tempting to judge this person by reputation or image, try to refrain. If you like and trust your flatmate then there's probably a good reason that he or she is attracted to this person and with a little bit of patience you'll probably figure out what it is. On the flip side, if you discover something truly disturbing about your flatmate's partner then he or she probably deserves to be told this information. It won't be easy, but think of it as protecting everyone's safety.