First of all, one of the most important things about touching a pony anywhere is the intent with which you do so. And I do not mean 'it is acceptable to grope somepony because you're curious' but more... If you intend to touch somepony out of affection and care, it will be soothing and pleasant. If your intent is of a more sexual nature, it will be pleasant, but certainly less soothing. This is how a pony may bond with and excite their partner with nothing more than a few little kisses or nips. Some areas are more... in tune with this latter kind of touch than others (some of which should be obvious) but, suppose a foal is seeking comfort and tucks against a mare's belly and teats. The sensation associated is more akin to a warm hug in a different part of the body than anything untoward -- even if the foal is not your own. Additionally how else do you think a massage with hooves could be pleasant?