was it illegal to watch porn in the 1950s?

No Henery Ford was the first true porn fan of note. 1930s. 

Porn films didn't really start being made until the 60s, and it didn't really become mainstream and available to regular people until the 70s. And even then you would have to go to a theater or watch in store (e.g. booths).

They might have had magazines in the 50s (although probably hard to get) but not really films/videos.

If you mean in the US, obscenity laws have been inconsistent and a lot of them were very local so didn't affect everyone evenly. Then there's always been that pesky things about defining what is and what isn't "pornography".

No but there were movie theaters that only showed sexually explicit movies. Those theaters would only allow adults to enter and were legal and protected by free speech laws.

Watch porn? You mean look at porn and that was only if someone could get their hands on a porn magazine and they were not available in the news agents

Porn at one time was played in theaters. I have a creepy uncle who use to tell stories of going to see "dirty pictures" as he calls them. I took me a long time to understand what he meant. I don't know if this was the 50s or 60s but it was a long time ago. The theaters were over in back ally type places and smelled of urine and other body fluids according to him. But, you had to pay to get in, I don't know about age limits to go in there, never asked and don't want to ask.

Porn in the fifties was limited pretty much to film and magazines. There was no videotape, no cable, or Internet.

illegal but widely available as stag films at legion halls

No but there wasnt no internet back then, mostly magazines like playboy and stuff.

There were no computers then. Porn was very soft core by comparison to today and some movie theaters ran them in bad areas. Some people set up huge projectors in their basement and ran old films that are laughable by today's standards.