I'm not into the specifics of https but i've been writing a lot of gateways for other protocols and it's never as easy as "just forward the message". Protocols some times have state that the proxy must be aware of and sometimes the forwarding is conditional which means that the proxy must understand both types of protocol and be able to act based on information in it. Say for example you want to block all requests to a specific resource, if your https server knows about this you might be able to reject the request before you decrypt everything of it.
