::It's because that is not what is referred to as Mammary intercourse or titfucking.  While, I can certainly see how language can be confusing this way, what you are talking about is known as "dove fucking", and usually refers to the rubbing of just one breast (at a time) against another woman's crotch.  In such a case, the woman is using her breast and nipple more like a sensation or penetrative toy.  That is also why there is not really any mention of rubbing the nipple against the penis in regards to this.  Titfuckin, mammary intercourse, or what-have-you generally is the action as described in the article, and if that action is not the primary effort of the activity, that is now how it is referred to.