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You've been seeing eachother more regularly. When you have an event or just want dinner, they're the first person you call. When you're in a group, people are beginning to ask, "So, what are you and xyz doing?" (Nosy, biotches!) You're obviously past the casual dating stage, but you've not had the boyfriend/girlfriend talk yet. The painful fact is that until you have decided to establish boundaries, there are no boundaries. You don't owe the other person anything and there is no reason why either of you shouldn't be seeing other people.
Many "official" relationships happen organically. You just kinda wake up and "hey, surprise!" you're in a relationship. Your friends know that you're together, their friends know that you are together, you act as if you are together, you spend most of your time together. In most senses, you are a couple...except that you're not. Without defining "that" between the two of you, however, you are nothing more than dating...openly. And, I can guarantee you, someone will be exercising that right. If you were serious about that person, you would want to make sure they were only seeing you and would take the step to ask them. If not, you would just let sleeping dogs lie and continue getting while the getting is good.
It's difficult to be the one who asks the other "what are we doing?" I'm willing to bet that if you polled a room of guys, that phrase would rank right up there with "it's that time of the month" and "no." My general thought is that dudes are territorial beings. If they really like someone, they will bring it up and EARLY. You might not make it official early, but it will definitely be part of the conversation. I'm not interested in entering something exclusive with someone that isn't that convinced about "us." I like a man with a plan...a plan that he wants me in.
People have different relationship comfort levels. If you aren't seeing eye-to-eye this early on, imagine what it will be like when you want to make it permanent (cough...marriage...cough). It's important to find someone who's views and approach to relationships are similar to yours. I'm not saying either of you are more correct, but you need to meet on that level. It would suck to feel like you were forcing someone to be your boyfriend or, even worse, your husband.
At what point is it necessary to have "the conversation"?