Are you aware that Google (Alphabet) owns YouTube and many websites display videos that are sourced from YouTube? I can't say that I have ever specifically needed to sign in to Google+ to watch a video on a different website, but that may because I am always logged into Google+ so that Google keeps track of which videos I have already watched and YouTube doesn't "suggest" those videos again in the "Up next" column on the right-side of YouTube pages. I'm not sure that anything needs to be "fixed".