I'm a 34 yr. old female, with a boyfriend of 1 1/2 years. I was married for 8yrs., but that's another story... I think that it's VERY HEALTHY to have friends of both genders. We get a unique perspective to our partner when we can have honest friendships with those of the opposite sex. I do think there are many men out there who WOULD have sex with their friends if possible; however, hopefully that's not the majority.
I have many male friends, some of whom I'm very close to. The difference is I'm honest with my boyfriend about my friendships--I tell him when I've talked to them and what we talked about. He doesn't feel left out, and I'm not doing it behind his back. They provide a viewpoint that is invaluable in my dealings with my boyfriend. And he has friendships with women too--two of his best friends are female. He has been close friends with them for about 3 years or so.
I do agree that you have to be relatively mature in order to have a relationship with the opposite sex that is nonsexual. And from what I've observed over the years, when sex (or fooling around) does happen between friends, there is a lot more to the story! Someone is feeling lonely, neglected etc. It certainly doesn't excuse the actions, but NO ONE PERSON can usually be blamed for a problem. If you have a strong sexual feeling for someone else other than your partner, maybe that's an indication to something that's missing. And yes, I have been in those shoes myself, but that's another story too....
Great topic--thanks for the opportunity to add my 3 cents worth.
Becky