This definitely reads like the work of the unmarried. First, you definitely intend to and want to marry your best friend. It’s a good idea. But the idea of best friends is something I always found limiting and a little juvenile. I literally referred to a dozen people at the same time as my best friend including my husband. They each played a different role in my life.
Right now, in my middle age, I have one other person besides my husband that is a best friend. At this point, I would drill down on that definition and say it is someone with whom I share numerous aspects of my self comfortably with trust and vulnerability. I do not think of my husband as a co-founder and I am not even going to explore whatever in the hell that means. It sounds too much like something a businessman constructed to discuss his entrepreneurial endeavor. What did we found? An apartment lease? A joint bank account? My stepson’s college fund?
The nature of a relationship does change over time. His “best” friendship with me seemed perfect and unassailable the first year. Then as you say, his son’s death battered at us. But we lasted. My health dragged us down. We hung on. Does that affect your sex life? You bet it does. That doesn’t mean you either stop being best friends or start clinging to one another in middle-aged desperation. You start having discussions of alternative sexual experiences. Because you’re best friends and you explore things together. It is just not like you have written it out to be. It’s more fun. It’s more communicative. It’s definitely less drab than I read it in your article. I would not want to marry anyone let alone my best friend based on how it sounded in the research I read here. Marry your best friend assuming you know that you need other best friends. Know that your sex life in the first two years does not reflect the match of your libidos seven years later, and ladies, his libido can be lower than yours. Every TV show ever taught me the opposite and did not prepare me. It can all be worked out.