If Google detects that content is available over HTTPS, by default it will index a website as HTTPS rather than HTTP, unless steps are taken to prevent this from occurring. This issue can occur if a WordPress site is set-up for HTTP and is using HTTPS for the admin area only, or if SSL is enabled on the hosting account but not used. Examples of using SSL could be running the entire site over SSL or running specific pages over SSL.
If you are running the iThemes Security plugin on the website, a quick work around is to apply the Secure Socket Layers (SSL) setting. This setting is located under Security->Settings->Secure Socket Layers (SSL). Under “Front End SSL Mode“, select “per content“. This will enable the ability to apply SSL settings to individual pages or posts. There should now be an “Enable SSL” checkbox on each page. At this point you can elect to enable SSL for specific pages such as a form if you like, but it is not required. iThemes Security will now forward a user to HTTP for any page that does not have the SSL setting applied. So even if Google displays the site in the search engine as HTTPS, the user will still be forwarded to HTTP. Eventually, Google should re-index the site as HTTP.
You may be able to accomplish the same thing using an SSL WordPress plugin.