The general forum problem with "website not secure" is that they have taken an extremely stringent view of things. In some ways this is good, but I wish they had used a wide range of messages to explain to what degree a website is insecure. This would have avoided panicking people.
Basically even if your site is secured with a certificate, using HTTPS, any content on your page that isn't secure will render your page as a whole insecure.
To illustrate. As I type this, this topic page is showing as "Secure Connection". But now I am going to add a link to an image from my own website, which is not secure:
Bingo! Simply by adding an image, I have rendered this page insecure.
So the security of your page is controlled by the least secure element on the page. Forums suffer disproportionately from this problem, as so many people contribute, some of who will be using HTTP links. Which causes people to panic and email the admins saying "your site is insecure", when the only thing insecure is that image.
Consider, that much of this concern could be avoided if - instead of "website not secure" - the message under such circumstances was "some elements or images on this page are insecure". it would save a lot of grief.
So there is not a lot you can do about the forums/"website not secure" problem, except stop people linking to http sites, which seems extreme. Eventually the problem will resolve itself, as HTTP becomes less common, but I think we are still a couple of years away from that.