I think as long as the opposite sex best friend is crystal clear, ie it has been said out loud and agreed by both parties, that the relationship will forever be best friends and nothing more AND if any party start to develop even the slightest feelings then they should stop being best friends then I think its ok.

For me personally, i live by the "Do unto others as you would have them do to you" thus if my wife has a male best friend I know I would feel uncomfortable so I don't do it to her.

Because we're raised to not trust. Parents tell us don't take sweets from strangers. They tell us don't believe what the Gomen and the mainstream media tells us. Society tells us don't have pre-marital sex because all the guy wants is sex. Newspaper reports of people getting conned by Nigerian men into sending them money with the promise of love or return in investment. Thus even though the the gf/bf/best friend wants to trust but it is hard wired into them not to trust. And thus the reason why you should always be as straightforward and direct as possible when you're asking someone to trust you.

Actually I believe everybody in the world could do with a bit more trusting in one another. Heck I broke up with my college gf because she didn't trust in me or her parents enough to introduce me to them. Being hidden (ie not being trusted) made me doubt myself and the relationship (am i not good enough for her?). Till this day she still has trust issues which is still manifesting itself in her current relationship (I still keep in touch every now and then (my wife knows about us keeping in touch ok! )).

How many types of "trust" are there? Trust is like a self defense mechanisme. it's ingrain. Only the level of trust differs from one to the other based on parents' teachings, life experiences and societal expectations.