I sometimes get the same thing happen to me.
Single, late 30's never been married.
But, i have had a few long term relationships (3 to be precise, one i lived with for 5 years, so i figure that counts)
The first GF was in university and i loved her more than life itself. I would have married her (at the time) but the timing was all wrong. Timing is a big part of life.
The third one i lived with i considered marrying, but she had a truly evil side to her personality. She could be the best, or mean and nasty and without a doubt, the most selfish person I have ever met. All along, there was this little voice saying "don't be stupid, don't do it" When you start fantasizing about dumping her every other day, you know it aint gonna work.
Thank Christ I didn't marry her.
I guess i want what I had with the first GF, and I would get married in a second flat.