I'm an orc, why can't I sleep in the longhouse if everyone else is?

Because you don't own the bed. Same as anywhere else.

Still, I'm an orc, I'm part of their community shouldn't I get a bed

You are an Orc, but you are not an actual member of the family/clan that runs the Stronghold. Malacath would want your character to marry some Orc females and start your own Stronghold, then when you are old, your strongest offspring can kill you and claim the Stronghold as thier own.

You can actually sleep there if you marry in.

Ah thank you, but there is no way to start your own stronghold right?

Not yet, but maybe as a mod or future DLC.