Relationships are tricky. We know this. Whether it is relationships with your family, friends or significant others, there is this gray area of overlap. As romantic relationships become more serious, it is sometimes important to evaluate how much should overlap.
Couple friends have become increasingly more noticable on tv shows and in movies. There was an episode on "How I Met Your Mother" a couple seasons back where Marshall and Lily were trying to find the perfect couple to become friends with. This is a part of real life.
Spending time with your significant other is great, but sometimes it is even better to have another couple to do certain things with. Couples vacations, double date nights, trivia night, the list is endless of things that can become more fun with other parties involved.
You can't just ask any random couple you see on the street to be your "plus two." It is important that a certain dynamic and chemistry exists between everyone. If your girlfriend finds the guy in the other relationship to be a jerl, or worse attractive, you should probably keep looking.
Once you find that right fit though, it is a great way to connect with your peers and experience more in your social life.