Okay, come up with a company policy for those (and if people feel strongly about it you probably want to know rather than have them just bitch in private about it), and if people get obnoxious about it do something about that. Instead of leaving your gay employee wondering if she needs to ask permission to mention getting married because that would be bringing up a "societal or political topic" (or to pick an example from elsewhere in the thread, if someone can mention that they went on a hunting trip on the weekend).