A good question,
I am not convinced that 'fun' encompeses the full reality of sex. I would suggest that 'fun' is what most umarried people or date budies are looking for, yet for the correctly ordained context of sex (that is between a married couple only) 'enjoyment' would be a more descriptive word.
That is enjoying each other and bring each other closer, in aphysical and very important emotional way, yes fun is involved,but, i would hardly describe the marital experince as just merely 'fun'! and it goes without saying, thats how babies become.