There seem to be only two camps of people when it comes to opera, and they are radically opposed. Those who love it are often very passionate, but those who claim to despise it appear to have the same amount of passion. It could be that this type of love or hate would drive people apart, but partners with an ability to share intellectually can find common ground. While one person in the relationship would attend the opera every night, their partner might wish to never attend a performance. Being able to find common ground in the stories told by operas, the emotional content of the music, or even the background of the composers could draw a couple together even if their passions differ.
The music of a good opera is full of emotional content, but many of the best ones are in a language not necessarily understood by those in the audience. It would seem to be a bar to keep enjoyment at bay, but understanding the emotions being portrayed is what many people love in this form of entertainment. Drawing the couple together through this facet alone could be what sets the two apart from other couples in the same dynamic.
Many operas are set within a particular historical period, and it could be another way for two people to share a performance. While the person who loves opera without restraint might be mostly interested in the music, their partner who loves history could find their own enjoyment in studying the costumes of a distant era. It can give them something to talk about after the performance.
Couples with all the same hobbies or avocations can find their time together becoming stale, but those who have differences can find ways to celebrate their individuality while sharing their passion. It can be the perfect mix on an intellectual level that brings enjoyment to both parties.