The difference between sexual assault and other forms of intimate contact is informed consent. If someone has intimate contact with you, in a situation where you did not -- explictly or implictly -- consent to it, it's sexual assault.
When I go to my doctor and she snaps that latex glove and tells me to cough, that's intimate contact; I don't like it but I know what's coming and, by coming to the appointment and not objecting, I'm giving my consent. When I do massage or acupressure on the gluteal region for a client (which I sometimes do for sciatica that might arise from impingement by the piriformis muscle), I inform them what I'm going to do and give them a chance to give or withhold consent.
But when a performer in the TSA's security theater, without warning, explanation, or option, grabs a woman's genitals and breasts, that is sexual assault. Read Erin's story and weep.