L
If I could give myself rep points for this post I would. I've been admiring it for a while - I think its my best post ever.
Anyhoo, I tend to agree with you that while in relationships, opposite-gender friendships shouldn't be the norm. Instead, I believe they should be the exception. I have two brothers (one older, and one younger), and when I get in a relationship or get married I'm not going to stop talking to them just because I'm married. Likewise, if I have a guy friend who is like a brother to me, I'm not going to stop talking to him just because I am no longer single. Since we aren't related, I would be careful to not hang out with the guy alone, but I don't think it is necessary to break off the relationship.
When people are in relationships, they can also tend toward befriending their significant other's friends as well, since they're all hanging out together.
I don't think a person should be required to end long-standing friendships just because that person enters into a relationship. I do think the friendship may have to change to accommodate for the relationship, and I do agree with you that the emotional needs of a person should be met by their significant other, and not by an opposite-gender friend.
I have a friend whose boyfriend has a girl friend he confides in alot. The girl is also friends with my friend, though, so she tends to tell my friend what she and the boyfriend have talked about. When my friend finds out that her boyfriend is confiding in the girl instead of in her she gets really frustrated and asks questions like "Why does he always tell her so much stuff?" or "why doesn't he just talk to me about this?"