Caught them 'at it!!'

[SoftInHead]

Son is 16, he had friend over to watch a movie. I knocked and went in and they are having sex. I said to get dressed and went back a few minutes later. I told them it wasn't on and they it was disrespectful and unacceptable. He just laughed and said it was up to him as he is 16. I said it's not on.
I went to the shop and when I came back I knocked and went in again and they had their kit off again. I said to get dressed. I gave them a roasting again and he just laughed. He said that I slept with my partner so what's the difference. He's not even in a relationship with this girl. I told him he's not bringing random girls here for sex! He just says I'm over reacting and actually laughed at me. I could have slapped him, I didn't tho. I am on my own with him and I can't even tell my partner as I am just so embarrassed that he would behave like this.
I just seem to go from one thing to another with him just now!

Has anyone else had to deal with this or have some words of wisdom.
Thanks x

I meant that immediately telling them to put their clothes back on would make the mum is question SEEM fuddy duddy and uptight in the eyes of a teenager. And I did follow up by saying there needed to be a no girls in the bedroom policy unless an established girlfriend. But the mum let them in there in the first place - it must have been pretty obvious they were going to have sex, and yet she walked in on them. 16 year olds do have sex, you know, and they must have felt pretty mortified. I'm not suggesting she should be the 'cool' mum - but you can't start laying down ground rules with the poor couple in flagrante delicto! So she needs to talk to her son, ban females in the bedroom, talk again about contraception and sex in an appropriate context between (ideally) a loving couple - but it's hugely hypocritical to pretend teenagers never have sex just for fun or out of curiosity.