As others said, best thing is to explain the whole scenario. My husband and I provided them with the date and circumstances of how we met online and also the date and circumstances of our first meeting in person.
The important thing is to make sure your explanation is clear and doesn’t contradict the timeline of your relationship anywhere else in the application. (In our case we gave a very clear and logical timeline of meeting online, meeting in person, living together, then married, which took place over a span of eight years. Our explanation showed a very obvious and logical progression of our relationship that made sense, which is ultimately what they’re looking for.)