misssclassy:
I won't. You want to know why?
This is why.
~A key factor for relationship sustenance is friendship and companionship. That friendship between the two of us cannot grow if he already has another lady with whom it he shares a strong bond of friendship with. He already sees her as his best companion so it would be impossible to ever see me as a better companion.

A guy who has a female as a best friend would share a lot of things in common and would have developed closeness with her so he would have to divide his time between the both of us.
She would be the first person he would like to talk to in matters that interest him. She would be the person he would want to share how he feels with.

He must have already told her a lot of secrets, some which I may never know. And since he has developed that trust for her, she would still be the person he would tell his secrets to in the future.
If we have relationship issues, she would be the first person he would like to talk to about it.
All these elements doesn't aid the growth of budding relationships.

And before you know it, comparison sets in. He starts to compare the friendship we share with that of with best friend. Then a loss of interest in our relationship follows.
Then a collapse in our relationship becomes eminent.

sweettease:
Not that I won't, but it's a turn off and I'll prefer the one who doesn't have a female as his best friend. I don't believe a girl and a guy can be besties and share no feelings, the romantic type.