“concubin signifie « celui qui couche avec “, taken from the link above, says it all.

A domestic partner outside marriage is referred to as a de facto husband or wife by some authorities

In Australian law, ‘de facto’ is the legally recognized relationship of a couple living together (opposite-sex or same-sex).

http://en.wikipedia.org/wiki/De_facto

In French, concubine has a male form, therefore he would be her ‘concubin’.

[Daphne – Fascinating: it’s actually the name of the legal status/regime which governs the relationship, and here’s the basic historic background http://fr.wikipedia.org/wiki/Concubinage%5D

Here’s a nice president. Actually a lovely couple.

http://tg24.sky.it/static/contentimages/original/sezioni/tg24/politica/2013/07/31/foto_giorgio_napolitano_vacanza_sesto_pusteria_01_1.jpeg

http://gossipblitz.it/images/gallerie2/NapolitanoGiorgioRelaxArgentario27Maggio2013/Napolitano-Giorgio-Relax02.JPG

[Daphne – Concubine in any language has both an embedded and overt meaning that places the emphasis on the fact that this is not a wife we are talking about, and which has strong connotations of inequality in the relationship. Therefore it is insulting whichever way you look at it. If Royal was Hollande’s concubine, what was he to her? There you have your answer as to whether the word is offensive or not.]