Best friend or not?

I read a book a few days ago. It was about this couple who knew each other since young, went on to marry other people, only to come back to each other after several years. The story made me wonder some things.

 

1. Is it actually possible for best friends to marry each other? Much as many couples say they are each other's best friends, I feel best friends are different from spouses. Till now, I don't think it's possible to have your best friend as your boyfriend. They're just different.

 

2. If you have a best friend who can understand you totally and care sufficiently for you, does it mean you don't actually need a boyfriend, since what the latter can give (emotional and mental support) can be received through your best friend?

 

3. I can start to imagine many of you saying boy/girlfriend is different from friend, so cannot replace each other. Well, what will happen then if your best friend is someone of the opposite gender?