Just because someone has a significant other, does not mean their lives are perfect or that they do not have problems.
I have a girlfriend, but I do not think I am good looking. I actually think I am very ugly inside and out, and I often wonder why my girlfriend is with me. This alone takes a toll on our relationship and causes lots of problems because I am always wondering: "Why does she like me?" "Does she really even like me?" "Is she thinking of leaving me soon?" which leads to me needing constant reassurance from her that I am "worthy" of her love. It also makes me jealous when she talks to other guys because I am constantly thinking that she will like them better and leave me because I am ugly, stupid, not fun enough, not cool enough, etc. etc.
I know these beliefs I have are irrational, and I am working on them with my therapist, but they are still really sticking with me, because I have thought negatively about myself for such a long time.
The only reason I have a girlfriend at this point of my life is because we met on the internet. I wouldn't have been able to meet anyone through friends because I don't go out that much anymore. But it's not like I magically was able to be all socially cool with my girlfriend right from the start. I was a nervous wreck when I first met her. I'm comfortable around her now, but we've been together for a few years, it took a while to be comfortable around her and not as shy.
As for your question though, I think that the idea that someone can't have Social Phobia because they have a girlfriend is really oversimplifying the whole problem. Because a lot of people are at different stages with their social phobia. I used to be really bad (couldn't talk to anyone, couldn't hold a job, etc.) But now I am a little bit better and can talk to people one-on-one. My biggest problem is meeting new people, dealing with people in groups and talking to people in positions of authority (like bosses). Those two really get me anxious.
But yeah, asking why a person with a girlfriend would post on a Social Phobia website is like asking an agoraphobic, "Why are you outside today?" Because we're trying to GET BETTER. That's why! It's the same reason some alcoholics still go to AA meetings every week even though they haven't had a drink in years. It's a long process to conquer a disorder like this, IMO.
The reason I post pictures in the picture thread and have a girlfriend and a job and what not is because I am trying to get rid of this stupid fucking disorder that has been ruining my life for the past ten years. Like I said, everyone is at different stages with this. Also, I feel people would be a little less judgmental of my pictures on a website like this. I wouldn't really want to post my pictures on a site like "Hot or Not" or something like that, for fear of what people would say about my looks.
But yeah, hope that answers your question.