What is special about physical contact in the context of erotic/sexual behaviour?
..."I never understand why physical contact is automatically associated with erotic practices"... We are not in control of how sensual desires arise. The only thing we can do to control erotic desire is to cut out what has been known to make the erotic desire arise and that means abstaining from physical contact. Doesn't it help meditation practice when we don't do things that could possibly lead to defilement and distraction? Erotic desire entangles  and greatly distracts us from the task of seeing things as they are. Metta :)