I asked Dear Abby who says that yawning is a show of indifference towards the person who we are talking to.
Some say it's ok. Other say it is rude and impolite. I favor the latter. I feel that when I am trying to express some idea, watching the listener yawn in my face, showing his rotten teeth, fillings and yellow tongue is the rudest thing to do.
Go to a job interview and yawn as much as you can and you will suffer the results.
When trying to interest a client of yours, yawn a few times... and he will lose interest fast.
On a date, yawn several times. You will not be given a second opportunity.
In Spain everyone yawns... and they excuse such behavior saying they are tired, they slept poorly and such.
Just in case, in the United States do not, I repeat, do not yawn. It is part of body language, and a negative trait, implying lack of interest.
Yawning is like picking your nose, your teeth, or wiping your nose with your sleeve.