Generally, a family unit is a man and a woman and any children they have. This can be broadened to include adopted children or a step-parent, but the boundary clearly excludes a man and a dog, for example.
Again: whose boundaries are these and in what ways are they binding?
Ok, lets look at it from the perspective of a group of people in a workplace.
Why should we? The situations are wholly different in terms of the nature of the relationships involved.
If they get on so well that they say, 'we are one big family' do they mean they are literally one family, in that they all have the same parents? No, such a 'family' is a copy of the actual family, which means a man and a woman and any biological children they have.
No it doesn't: all you are doing here is highlighting that colloquial English allows words to have different contexts - 'family' is but one example, and the obvious other one in this context is 'gay'.
So two gay people can not be a literal family.
Says who?
There is a distinction between a biological family and a non-biological one.
There may be a distinction but in terms of the immediate ancestry of those involved but then spouses are rarely closely related in biological terms. So, for example, are you saying that when Mrs G and I got together and married we weren't a 'family' for the 5 years before our first child arrived?
Fine, so why not include gay couples in the broad definition of family?
Why not indeed: I'm happy to do so.
Because they do not have the potential to ever be a biological family, just like a man and a dog.
Yikes - this reads like you are equating same sex or mixed sex relationships where children aren't an option with 'one man and his dog'. I'm sure this isn't your intention but it does suggest your thinking on this issue is all at sea. In addition, on whose authority is it that marriage should involve the potential to be a 'biological family' predicated?
This is how the boundary is set.
Again, by whom?
That's why it would be absurd to allow humans to marry animals, or adults to marry children.
We don't allow either (assuming by 'children' you mean those under 16 years of age) - but that is a different matter to allowing adults who are free to marry to do so without interference.