Phone seized at USA airport please advise : legaladvice

I flew home from Ireland to Boston last week. This was the end of a four month round the world trip starting in Japan. I am American and white and was born here. This was my first time outside the USA since I was 7 years old.

I told the officer I had been to Japan, China, Thailand, Sweden, London, and Ireland. He asked why I went to Thailand and I said for the beaches. He sent me to another room to wait for another officer.

The new officer asked me the why I was in Thailand and how long I was there and asked to look at my phone. I said why. He said because I had been to Thailand and sometimes American men who go there are involved in drugs and underage girls.

I unlocked my phone for him and he looked through it for fifteen minutes while I sat in a room and found some homemade porn that I made twos month ago in Thailand. Big deal. He then asked specifically how old one girl was and I said 19. He said he didn't believe me and he thought she looks more like 16 but they are Asian they all look younger.

He took the phone and wouldn't give it back. He said they were going to copy everything on the phone and send it back but he did not say when.

Is this legal? I don't think he had a good reason to be looking through my phone at all. If they do find anything illegal on the phone (I don’t think they will) do I have a good chance to get the case thrown out?

I don’t think there is anything illegal on the phone but he seemed really intense about the video of the 19 year old woman like it was a crime or something. I did chat with some other girls there on Badoo and one of them sent me nudes and I can't be sure how old she was but he didn't mention those.