Most Helpful Guy
It's not true that it automatically means they are cheating.
It can also just be a sign of insecurity, paranoia, or that they are cheaters themselves (they may not be cheating on you right this second) and believe that it's impossible for someone to stay faithful. Hence they are always doubting you.
In my opinion, I don't care what the source of their accusations are. Such people are poison, regardless of how they got infected. Dump them fast, and dump them hard.