I recently had a long-haul overnight flight from the US (10 hours approx.)
Once we had eaten/had drinks, I reclined my seat and put something on the TV. The man behind me immediately began tapping me on the shoulder and when I turned, he asked if I could put my seat into the upright position as he had a drink on his tray table. I agreed to this.
Around 30 min later, I was ready to sleep so I told him I was going to recline my seat. He asked me not to, as he said he was very tall so needed the extra legroom... I thought it was unfair to expect me to sit upright the whole time, and he could have booked an extra legroom seat (which, incidentally, I had). Also, he could have just reclined his own seat to give him the same amount of space.
In the end, after some back and forth, I agreed to recline it a bit but not the full way, but it wasn't as comfortable as it would have been if I'd reclined properly. Also, I thought he was a bit unreasonable to expect me not to recline so I could sleep, basically to sacrifice my comfort for his, but I wasn't prepared to spend the whole flight arguing so wanted to compromise.
WIBU?
YABU - you should not have reclined your seat
YANBU - you were perfectly fine to recline