I think it is very hard for two people of the opposite sex to "just be friends". If the two are attracted to each other enough to be friends, the sexual element usually creeps up as well. It helps if both are in a committed relationship, but if one or both are not. . it's not surprising that feelings happen.
It also depends on how much time is spent together. If they are best friends and both single. . well. . I don't think I couldn't keep my hands off.
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My husband pretty much doesn't allow me to have male friends, well, he'd never let me hang out with a male alone. He thinks that because I am totally beautiful to him, that all men want me. . ya know what, he's proabably right? J/K
And no offense Watson, but I don't think people can "completely" trust their mate. Jealousy exists, and it's healthy.