Is that true?

So today I was in the bedroom and my so was in the living room watching tv... He was in there at least an hr or he came in to cuddle and I notice he had a boner! I was like why do you have a boner. He said because he had to go pee and was holding it. I don't believe that! I don't think I have trust issues with him. But I'm always questioning him because he is a very compulsive liar! Just asking is it true that holding in your pee can give you a full bone boner? & should I be mad about it? I just think it's wrong...