Whilst I can see the point to an extent of the challenge, if heterosexual couples live together and wish to make provision for each other, they can always enter into a "Living Together" agreement which sets out the financial responsibilities of the one for the other.  It is usually used in the negative sense i.e. setting out that neither has an entitlement but I do not see why it could not be used in the positive sense.  The difficulty of course is that whilst it might be a contract between the parties, there would be no specific law of enforcement, as there would be with civil partnerships, and thus enforcement might be difficult.