What would you do...

Shut the door and never mention it ever again? It's what I hope my parents would do. >.>

Penis

UHM.... >;( WHAT U DID ISNT RIGHT

I would lock the door to prevent that from happening or at least delay it.

I would do the girl.

I aint no mother :o or am i....

join in pretty sure that was said, why did you run into your son with a girl

"Silly amateur, you're a disgrace for your family"

say you're not doing it right, look at how bored she looks

''SON, I am disappoint.''

Allow me to join.

Say "Your dick is so tiny, lawl"

Well, I would say "Oh honey, what have you done? now the aids will be in that cute girls body too !"

are you grounded Dr?

...for a klondike bar?

jesus christ, op is talking about his minds or experience >:o

I would tell my son to make sure he has condoms. And then I would be sure to knock in the future prior to entering his room.
(I'm also a mom with 2 boys.)

i would open a topic like this.

Would tell him, you are doing it wrong, my dear little son, put it in da butt, in da butt.

Say "Oh, hehe. Go on!" and leave the room, probably.
I seriously don't see what else you would do. People act as if that's something bad or even unusual while it's the most normal thing in the world and the sole reason this son I just walked in on even exists. Walking in on him masturbating to some weird-ass shit on the internet would be much weirder and I think I would be cool with even that. I'm pretty open when it comes to stuff like that anyway. In a "whatever floats your boat, son" kinda way.