*ahem*
To answer your question, OP: I'm not really sure. My gut reaction would be to say that we're in a heterosixual relationship, but for all we know, Howard/Nora may actually be bisixual (as all your human companions are, as they all can be romanced) and just felt like marrying the opposite six and siring a kid. It's what I plan to do with my first character.
Good question. Unless explicitly stated, Howard/Nora could just be a friend/sibling who is helping the other person care for the child. We just assume they're a heterosixual couple because...well...that's just what we assumed (probably because it's so prevalent in media.) I'm sure Bethesda will let us have some wiggle room so we can roleplay Howard/Nora's gay best friend/sibling who decided to help raise the boy.
OK, my first character has been decided. A bisixual man who decided to live with Nora and help her raise Shaun.