* Say no to date in person
* Decline the job for reasons that you explain to the HR department: He asked you out on a date in a professional setting (I must inform you .... I regret ... best regards). Writing this in a friendly form (the HR person receiving this mail has not asked you for a date). This person will be up in arms for you. You will receive a very very friendly call from someone up the foodchain. See, it happened in person .. so you cant prove shit. But if you are a boss.
* Do not send the mail CC to the interviewer
* Close the mail friendly with best regards
* Document what you can. I guess the interviewer did not do this in written form.
* Go on and strike this company off the list of companies that you aspire to work for. Do not believe this was an single incident. When hiring people everything should be extra shiny (normally you discover problems after starting).
So with 15 years experience in the field: that shop is crap!
One thing: The tech job market is a sellers market. More people want to hire than there are able engineers. You are not the one asking for something only. The company is as well. A work relationship must be on the basis of a mutual understanding what is ok and what not. Seems you have a difference here. Good luck for the job hunt. The good companies are out there (and many people who can show you the way for harassment free work experiences).