Brutally Honest in a relationship, is it necessary?

No, it is not necessary

You can be honest in your relationship, but you can’t be brutally honest. If you really care about your relationship then don’t become “Sheldon Cooper”, because not everyone is “Amy Farah Fowler”.

There is a line, where you can’t be honest and where you must be honest.

Que: Are you cheating on me?
Ans: If it is yes, then you must be honest with him/her. Because it is their right to know whether you are cheating or not. They have chosen you, they could have been with anyone but it is you who holds special place in their life.

Que: Am I fat?
Ans: Even her weight is 100+, you don’t have to be honest here. At least she wants to look pretty in front of you. There is no point to break her heart.

You can clearly see the difference in above two question. In first question, if you lie then you are wrong. And in second question, if you don’t lie then again you are wrong.

There is a fine line between a brutally honest and an asshole. It is up to you, what you choose.