If i look in my google profile, at the list of services authorized to access my account, most if not all sites that uses open ID for login only require 'Sign in using your google account'.
Your site requires more - 'Google+ You'.
If you truly only need openid login (you have marked http://theoldreader.uservoice.com/forums/187017-feature-requests/suggestions/3743312-allow-me-to-authenticate-via-google-without-giving as completed, after all) please only require that level of access. It is good that you stopped requiring access to contacts, but not enough.