The wife (usually) who attends a work function with her husband is an awful thing, or at least something from the last century. It's not just about whether or not a person is heterosexual. You should not be defined by what your wife, husband, partner or anyone else in your household is or does. Sarah Vine is a journalist who happens to be married to Michael Gove, and should be noted as a journalist, nothing else. People may be having relationships problems, some are divorced and it is no-one's business at their workplace. For the person who is not at the workplace, inevitably such events will have conversations about work or whom they work with, which to an extent they are excluded from.
The answer could simply be 'they have another engagement that evening', if you wish to be polite.