When I stream porn in my hotel, do they know?

Anything done on their wireless they have the potential to know but no one much cares.

You probably have a smart phone with all kinds of apps, including Google ones, and you're worried about the hotel WiFi? They have better things to do than to inform your employer that you streamed "Backdoor Adventures of Barely Legal Twinks 3" while staying there on a business trip.

who cares?

Yes, and they're watching you watching it. If your room has a light fixture, smoke detector or air vent, cover them up before pleasuring yourself.

I can't imagine that being legal R4.

All staff members cram into the manager's office and they go through your browser history for a good laugh.

Make sure you wear a tinfoil hat while you masturbate, OP. Also, wearing oven mitts at the same time is advisable.

Put a little tinfoil hat on your other head too, OP.

Not an issue, but the hotel does have complete visibility to what content you've viewed on your in-room viewing. Countless people rent porn, watch and jack off after 2 minutes, then complain to the front desk "I rented Frozen on-demand and it stopped playing". They'll often refund you because they want your business, but they know exactly what you watched and for how long.