One doughnut will not make you unhealthy just the same as one salad will not make you healthy.
Dealing in absolutes is never good. It is the total intake of your diet over the course of the day/week you should be concerned with.
One meal will be redundant, it is only a very small part of your overall intake. What you should be concerned with is the overall quality of that total intake.
Instead of trying to be perfect with your diet but only achieving it 30% of the time. Just aim for being good 80% of the time.
You aiming for being perfect will only result in you snapping and coming off it when you eat something you deem ‘bad’.
Go for consistently good, not inconsistently perfect!