It's definitely untrue that men cannot be mere friends with (attractive, is what I assume we're talking about) women without an ulterior motive. The majority of my friends are women for whom I have no interest in beyond friendship, and even the few that I would be interested in more with I would still be friends with regardless. I wouldn't be attracted to a woman if I didn't get along with her on a personal level (i.e. friendship) anyway, so it's basically impossible (for some anyway) to be friends with a woman simply because I have some deep-seeded hope that I'll some day have a shot at her. Just because a woman is attractive doesn't mean she can't also be a friend. It's alarming how simple that statement is yet I feel that it needs to be stated anyway. I'm not speaking for all men, because I know that there are in fact a lot of stupid men in the world who are incapable of dealing with the opposite sex in any way that doesn't eventually involve a penis, but I'm sure there are plenty of men who are capable of actually being friends with an attractive woman instead of pretending to be friends while crossing his fingers behind his back in the hope that he'll eventually get in her pants.
Besides, there are plenty of reasons that a guy wouldn't want to be in a relationship with an attractive female friend, like personality clashes, if she has children, religious disagreements, or any number of irreconcilable differences that one can foresee potentially destroying a relationship.