Yes it can be normal for some people depending on their phisiology and what object is being used or how much lube is involved.

You may need to find a plug with a different base that will help stop this for you.

You may need to use eaither a smaller or larger object or a different shaped one.

Your body is most likely trying to tell you something if your not maintaining arousal during it, you may eaither be doing something wrong, or your body simpley doesnt get turned on physically by what your doing.