Is it true that if a guy and a girl become close friends, the guy always wants something more?

Not always my uncle has had a best friend who is female, since childhood,  they are both in their fifties now, the idea of them dating eachother was laughable to them both.  Also, what's to say the girl will not develop feelings for the guy? The risk is there if one finds the other attractive, but women can also be drawn in by a guy if he is a really lovely person, a person's character can make them more attractive than just the initial aesthetically pleasing.

There's so many cases that close friends turn into relationship but the statement that a "Guy" always want something more is highly false, but some born to be like that on purpose which it's another story but i'm not like that & I will never be. 

In any relation there's limits & lines of respect, whoever don't know those limits & lines are immature & as a matter of fact - they're losers.

Not necessarily the guy. But if a girl and a boy are close friends mostly it turns into a relationship, in case they both are willing to do so.

In my opinion best friends make a really cute couple together.

May be it is true.

Some guys. I've read a saying in the book 10,000 Ways to Say I Love You that goes like this: "A guy is already half in love with a woman that understands him."

In general terms ? .. No, of course not.  There is no singular behaviour or tendancy that pertains to ALL members of any gender ALL of the time. However, plutonic relationships are quite common.

Guys seem willing to want sexual favors with most willing females they have an attraction to, or will think more selfishly even if there is no attraction, but some guys though they might want to will still have self control and that's commendable. 

Its an individual thing same as for females, Its about integrity.