No, you don't HAVE to lean forward with an internal. It can have a bit more inclination to encourage it, but you can overcome it without undue effort.

Putting a load on your back (in a pack, a gunny sack, or whatever) shifts your center of gravity back from its "normal" location - and to compensate, you feel like you need to lean forward. After you get used to the new normal center of gravity, that sensation goes away.