Movements, facial expressions, tones... Those mean anything? I can tell you right now I make 1 million dollars a year, drive a porsche and have a house with a six car garrage, but that doesn't mean its real. In real life, I could tell you the same thing, but judging from all the above you could probably tell I am lying. Although those seem to be blatant lies, you cannot tell as easily as you would in real life. You might think the relationship has deep bonds, but how deep are they really? Ask the person you like in game(I am generalizing the people who met in games then began a relationship) to meet outside the game and most would be very hesitant or even refuse to do so. A few might even have love interests outside the game making you nothing more than an in game fantasy. Attraction to people begins with the physical and then moves on beyond that; however, in game you cannot do that. If you were to meet someone outside the game and they were not attractive, you would probably not pursue the relationship any further. Physical attraction is just that important.
Its true, people can start relationships inside of an online game and something more can spring forth from it, but those people are very limited and very lucky. It takes a good leap of faith to trust someone in a game who could very well be nothing like what you imagined. I've met people from online before, a majority of them were nothing like I imagined or said they were and a few were exactly what I imagined(this was not a good thing either). Those that talked big weren't as tough as they said and the ones that acted like asses were just as annoying in real life. I see people say "You can tell what a person is like from how they act online" or "people act the same online as they do in real life" and this is nothing more than a lie. People act online the way they want you to think they are in real life and that is the truth. It is true, some parts of who you are do leak out, but someone else would have a hard time desciphering which of it is the real you.
A single common interest is not sufficient to sustain a relationship. My parents are both together and its very rare for parents to stay married now(I believe divorce rate is now at 51%). I know for a fact its not because they had one thing in common but rather a lot more. Although your parents had a common interest that is not what kept their marriage strong, its the common interest that started their interest in each other and blossomed more interest. Had it not been for the many other factors besides that one, your parents probably would not have fallen in love.
Starting a relationship on the internet has many disadvantages. Distance, lack of contact, language barriers, among many other things. Long distance relationships are always bad. The lack of physical contact between two people can tear an otherwise healthy relationship apart and being online one almost guarantees to be long distance. Time differences also come into play. Someone who lives in the US can have a difficult time interacting with someone who lives in the UK. These are just a few of the problems you face that a normal relationship does not.
I am not dismissing online relationships entirely, its just that a relationship has a very difficult path but to add to the problems faced, almost dooms it to certain death from the very begining.