Heh...my Dad used a slightly different approach when he proposed. He told Mom "Either you say you'll marry me or I never want to see you again - we can't be just friends."
Apparently it worked. She said yes, and they had been married just shy of 33 years when she passed away.
I'm sorry about your moms death. I don't have any male friends and my husband doesn't have any female friends. It works for us that way.
Sometimes, I think the "let's be friends" line is like a last test. The lady is waiting to see if the man wants her badly enough to kick it aside and sweep her off her feet despite her silly hesitations.