In August my parents will be married for twenty-five years.

The phrase "will have been married for twenty-five years" correctly indicates a future perfect continuous tense, which is appropriate in this context. It conveys that at a specific point in the future, in this case, August, a certain duration of time (twenty-five years) will have been reached in the marriage of the parents. This tense emphasizes the completion of an action over a time period leading up to that future date, showcasing the longevity of their relationship.

Using this construction allows for a clear understanding of the timeline: as of that future date in August, the condition of being married for twenty-five years will not just exist but will have been established by that moment, making the sentence precise in its meaning. This is crucial when discussing life milestones such as anniversaries, where the emphasis is on both the current state and the historical duration of the relationship leading up to it.