Whenever I hear someone refer to their husband, wife, spouse — even their Significant Other, a phrase from a now-distant past — as their “partner,” I think of something Wendell Berry wrote decades ago: 

Marriage, in what is evidently its most popular version, is now on the one hand an intimate “relationship” involving (ideally) two successful careerists in the same bed, and on the other hand a sort of private political system in which rights and interests must be constantly asserted and defended. Marriage, in other words, has now taken the form of divorce: a prolonged and impassioned negotiation as to how things shall be divided. During their understandably temporary association, the “married” couple will typically consume a large quantity of merchandise and a large portion of each other. 

“Partner” is, in the context of marriage or even long-term cohabitation, an ugly word, connoting as it does a business relationship for mutual profit, ready to be dissolved when the profits aren’t high enough. It should never be used in the context of mutual love.  

UPDATE: My friend Andy Crouch has written to me in defense of the word “partner,” suggesting that it “has a wider frame of reference” than I allow, and pointing out that it’s the nearly-universal translation of koinonos in Philemon 17. This is a very good point! I’ll take this under further consideration, but for now, several thoughts:  

1. The word certainly had a wider referential scope in the past. If you look at the OED you discover that Milton’s Adam says "I stand / Before my Judge, either to undergoe / My self the total Crime, or to accuse / My other self, the partner of my life.” And Robert Southey 150 years later: "So forth I set … And took the partner of my life with me.” 
2. However, this kind of usage almost completely disappears for nearly two centuries, until it is revived largely by people looking for a word to describe committed gay and lesbian relationships, at a time when such people could not marry. But up until that time, again if the OED is any guide, the business-based meaning had for many decades almost completely displaced all others. 
3. Thus one could reasonably conclude that the business-based uses of the word have become so dominant that they cast a strong dark shadow over any current use of the word — which is my view. Or recent uses of the term certainly could reasonably be heard as a renewal of older, more richly human meanings — which is Andy’s view.
4. So the connotative situation is definitely more complex than I acknowledge above. 
5. The economic overtones of the word would certainly be displaced if one were to follow Milton and Southey in making it a phrase, “the partner of my life,” or, more shortly, “life partner” — but that, I suspect, is a phrasing most people who employ the term wouldn’t want to commit to. 
6. Finally: I wonder if, given the connotations the word has acquired, “partner” is a good translation of koinonos, or whether an alternative needs to be considered. I notice that the 14th-century Wycliffite version of Philemon 17 has “Therefore if thou has me as a fellow, receive him as me,” which captures the idea of koinonia as a fellowship — but we don’t use “fellow” that way any more. Maybe contemporary English has no real equivalent to koinonos. That would be a situation worthy of our reflection.