SSL has noting to so with SEO. And most certainly will not effect SEO. I swear not everything on the web has to somehow be "SEO" related.
Now, as long as a form submits to a page that is behind a secure connection (HTTPS) then the forms contents is encrypted before sending. The form itself does not need to be on a secure page. However, if you have the ability to enable SSL for all pages, you might as well. (Now unless you have large amount of traffic...thats another concern, SSL will require more processing time for the server.)