If it's on the internet, it must be true. No, seriously, a friendship is already a relationship, and depending on its particular circumstances and/or the particular people involved in it, having sex has the potential for ruining the friendship.
Good for starters! That is a particular kind of relationship. But really anything between people is in relationship one to the other so is a relationship. And all perhaps forms of love which can only exist in relationship. Perhaps the net article is thinking in terms of OK they are in a relationship meaning they are dating or seeing one another or married etc. Or, on the other hand, someone might say oh we are not in a relationship. But of course as long as they know and interact with one another they are in relationship, so in a relationship of sorts. But we can be friends and lovers and not be committed to one another so that would not be a "committed relationship". Which to me means a close and ongoing primary relationship. But we can be committed to one another and not be having sex though that seems rather a waste to me. Perhaps some people can have more than one primary relationship like in a group marriage kind of arrangement but for me I can have one only though I might have more than one secondary relationships. Which with me are primarily sexual in nature and though we are of course friendly we are only sometimes really friends. I guess so weird how I can be very sexually intimate with many people for even extended periods of time without coming to know them very well to any depth. Because there is not the time for me to put in to them as I am committed to someone else. Great sexual relationships are very wonderful but relationships between friends even more so because they transcend sex though they may include it as well.