Unrealistic? I wouldn't say that in that person's mind that it would be but it is not something that anyone else can do for you. I know this first hand. I was always insecure about everything the majority of my life. In my mind, someone/anyone could help me out of it. In actuality, I had to find it in myself.
Even the beginning years of my marriage. I married him because I trusted him, yet I couldn't completely "trust" that he wouldn't move on. That I was really lovable. That is where the problem was. I didn't think I was lovable. So, why would he stay with me? After all he was a musician and had women throwing themselves at him all of the time. What was to stop him from breaking down with that kind of temptation?
I wanted and needed "HIM" to reassure me. Yes, he could say it and at that moment it would mean something but I had to believe it myself. I had to realize that I was lovable and that his commitment to me was because I was lovable. That he did in fact love me. Therefore I was worthy of love.
So to answer the question it is not unrealistic to want it. It may be unrealistic to expect it. The partner may be unable to supply it. The partner has his/her own issues and this may be one for them. We are all insecure in our own ways, expecting something from someone may be unrealistic because that person may not be able to give it.