I'll tell you a story. It's a story where I did an incorrect thing, but hopefully it will explain why guys do this particular thing.
I was seeing a girl, and our relationship was pretty good. One weekend I was taking an out of town trip to a convention and was going to stay in a hotel. Convention went great, but there was a problem with the hotel and suddenly I had no place to stay. If you've ever been to a big nerd convention, you know there's zero chance of finding an unbooked hotel, though I did look.
Instead, a gaming buddy of mine who lived in that city and who was at the con offered me a couch. Her couch.
So here's my situation. It's late. I'm exhausted. I have an early day the next day. I have a free couch on offer from someone who is just a friend and nothing at all is wrong with that. But I also know my gf has trust issues, and if I send a text at 1:30 AM that says "hey, change of plans, I'm crashing with my friend jane because they overbooked the hotel," I'll get a sobbing phone call that will last three hours. At best. And I just didn't have it in me to do that, so I just didn't say anything. By the way, that wasn't an assumption - last time I had done something similar when plans had changed and I ended up being one of only two people who didn't bail on movie plans (the other being a girl) she freaked out so bad I had to bail on the movie, drive an hour to her house, and stay on the phone the whole way.
Was I "hiding" something from my gf, being "dishonest?" Yup. And I'm not saying it was morally correct. In fact, there were clearly reasons we needed to break up. But it wasn't because I was shady - it was because she was bananas and backed me into a corner.
OP's BF should have done what I should have done - broke it off instead. If you have to hide perfectly innocent stuff, then your relationship sucks. Instead of doing that, you should just end it. But like me, it's possible that OP didn't want to do that because maybe things are good most of the time and this seemed like a relatively small thing overall. But then it isn't.