Narcissus and monogamy
The relationship structure of the Narcissist is also due to the fact that monogamy did not develop until a rather late phase of evolution and is still established only among male with low social status. It did not yet gain ground among Alpha male. Or those who believe to be. Should Narcissus exceptionally remain loyal to someone for a long time, then only for lack of alternative opportunities or because of the threat of grave sanctions, especially those which could damage his image.
Speaking of social status: The Narcissus is seeking suitable partners. Which means: Partners who are inferior in respect of intelligence, education and social background. There is a very simple reason for that preference: You can control them better and manipulate them easier. This has clear advantages. For both.
The Narcissist has to be fed and he needs his ego extensions like the air to breath. In return, he ensures that his closest environment is doing well. Only for the reason, that he himself feels fine. In this sense, he is quite capable of exuberant care. The more as the care he gives to others, serves his own well-being. Even after his own death, he still wants to make a good impression. Fortunately nowadays the trend is rather away from pyramids, tending to legacies, which are of immediate, pecuniary benefit for the survivors.
The respective partners at his side often appear strangely arbitrary, indiscriminate and somehow interchangeable. Main thing: Presentablility. That's because Narcissus would rather marry himself, anyway, if he could. Which is not so rare as You might think. In fact he really only needs a partner because he
1. has certain needs and for the continuous satisfaction of those he requires an ego extension,
2. needs good PR,
3. might like to reproduce itself.