Your on the money there.
The reason why that happens is that girls instinctly look at guys as providers and when your already with a girl that automatically shows that you are a good provider and makes you more desirable to them.
Believe it or not but some people believe this comes from thousands of years ago when the men use to be the sole providers, food, shelter, etc.
Your looks don't count for that much when it comes to women, unlike us guys we base our decision on a partner on about 90% good looks (this is also believed to be genetic).
It could also mean that the sensed your desperation on you when you were single and now that you are taken and don't care...
Its an interesting subject and maybe it is in the wrong section?