The short answer to your question is to provide accurate and honest information on your mortgage application. If your personal circumstances change after your mortgage closes, such as if you get married in the future, then that does not invalidate your mortgage or change your loan terms. There is no waiting period after your mortgage closes before you can get engaged or married or make any other significant life decisions such as changing your job. Please note that if you do get married you can add your wife to the property title but you cannot add her to the mortgage, unless you refinance your mortgage (this applies to all mortgages, not just USDA home loans). So you are solely responsible for this mortgage even if you get married in the future.
It is important to emphasize that you should wait until after your mortgage closes -- and not just until after you have submitted your mortgage application -- before you make any major personal or financial decisions. If there are significant changes in your personal or financial profile, such as getting married or applying for a new car loan that causes your credit score to drop and impacts your debt-to-income ratio, this can undermine your ability to close your mortgage.
The only time an issue arises with a USDA home loan, or any mortgage for that matter, is if you provide inaccurate or dishonest information on your mortgage application. For example, with the USDA home loan a borrower may elect to omit a residence of their household so that they do not exceed the USDA borrower household income limit. This is an example of mortgage fraud, which can result in your mortgage being invalidated or potentially worse.