Would you enter your contact data on a website that was unsecured? We wouldn’t.
https is a secure way of transmitting the data associated with your website. On most browsers (even those on phones), you’ll see a lock icon in the address bar. The https lock icon appears when your site has an SSL (secure socket layer) certificate.
If your site collects personal information you should be protecting the data of your donors and supporters with an SSL certificate.
Usually costing approximately £50 per year and a couple of days to implement, your website hosting company will almost certainly offer an SSL certificate and set your site up with it.