you're not going to like what i have to say, but you asked "why" this can happen. first, sexual assault is cultural - it differs from culture to culture. remember clinton saying that he did't have sex with that woman? by the cultural standards applied, oral sex is not sex.
several yrs ago in saudi arabia a teenager was taking a sun bath in a two piece swimming suit behind the 10 foot wall in her parent's house/compound. a man crawled on top of the fence and watched the girl. the then raped her. they went to court. he was beheaded for raping her and she was given 10 lashes for enticing him to rape!
men get away with it for the same reason rich and powerful people getaway with things: because they CAN. there is noone to stop them.
by the same token, take the other side. during the 60s there was a case in which a man was riding on a public transport bus in LA. a young woman sat down next to him. she told him that if he didn't give her $50, she would yell rape. He didn't and she did. he was found guilty (must have had a public defendent lawyer) and went to prison for 10 yrs.
there has to be a line somewhere. i just don't know where it is.