is it legal to discriminate against non family groups?

Discrimination is illegal only for a very few specific protected classes like race, sex, disability, etc. It is totally legal to discriminate based on being a family unit or not.

I'm not sure if you mean 'partner' as in someone you do business with or a gay partner in which case you might be talking about discrimination by sexual orientation.

I do know that MANY zoning laws for residential neighborhoods require people living in a house to be related to each other, though. That is why they are refered to as R-1 zoning - residential single family. They don't want the houses run down as flop houses for as many as can be squeezed into them.

Actually yes that is discrimination. I'd speak with a lawyer and confirm it for your area but here it's illegial to reject people from housing due to such things. You may even be able to take legal action against the property manager and/or owners.

Yup, legal as can be.

There are no laws protecting singles or groups as a class.

Their house, they can lease it to whoever they want to lease it to

Sorry poncho. Start looking again.

yes if you are white. no if you are not white..