Those of us who come from cultures that are more diffident than demonstrative are surprised when we find that many residents of Mexico City are amorous exhibitionists.
Any public place is fair game -- a square, the metro, a park, a bench. Their backs against a tree, a stone wall, a plate glass window. There may be many reasons for this but I have no doubts that the main motivator for public displays of affection -- the main motivator for so much of Mexican society, really -- is poverty.
These are people who live with their families so they have no privacy at home. They are out in the open because they have nowhere else to go. You may feel like shouting "Get a room," but if you do, it would be advisable to give them a couple of hundred pesos so they can pay for it.
Still, these demonstrations of PDA are one of the elements that make this such a sexy city. El D.F. may not be in-your-face sexy like, for example, Rio de Janeiro, and the citizens may not be as similar to Vogue models as, say, those who live in New York or Milan. But chilangos cannot keep their hands off each other and that can be pretty hot.
I would like to say that I purposely took these pictures out of focus or with the subjects facing away from the lens because I was conscientiously discreet. I am afraid I that would be a lie, though. I'm just a lousy photographer.