I think they can be friends. My 'best' friends are mostly men.. apart from my little sister.. and my kids..
The territory changes when one of you gets involved with someone else- but it's like Darude says; if you both have respect for each other and to their priorities, it usually comes good.
Friendship after a relationship with a member of the opposite sex can be doable too.. with space. I always feel that if you've shared such a chunk of yourself with someone, they're worth investing patience and time on.
Gosh, that was deep. phewwwwph.