My wife prefers long-term relationships as well. Over the past 10 or so years she has had 5 lovers. The first 2 were very short term. However, the last three have all been long-term.
Her third lover lasted for 18 months, her 4th for 4 years and her current BF has been on the scene now for 4 years.
They have both fallen in love with one another so it is more a poly situation. I introduced him to my wife with the clear understanding that if they clicked then sex was more than OK. Before they met I have told each of them that I was hoping that their relationship would become intimate.
The first time we met together I said that I wanted to make it clear to them both. If they both wanted then I would love their relationship to include sex. They clicked, sex happened and the three of us are very good friends.
We even talked about living together buy logistics makes that extremely difficult.