It is nearly impossible to make these sorts of arrangements absent a legal marriage. Marriage isn't a pre-requisite for owning property together, but if you get married, in most situations in most states your property will be jointly owned regardless of who pays for it. This is the reverse of the presumption that applies to unmarried couples. Divorcing couples are also entitled to demand alimony if the marriage doesn't last, without the need for any explicit contract providing for post-separation support. Every marriage requires a formal ceremony, and every marital separation requires some kind of formal court action, and quite often the help of a lawyer.
Unmarried couples can break up informally, on their own terms.
Absent a legal marriage, a couple needs to sign several agreements to create even a partial framework of protection in the event of death, and certain tax benefits are forever denied to unmarried couples. If you are married, however, the surviving spouse generally inherits all the property if the partner dies without a will. At death, a bequest from one spouse to another is tax free, regardless of its size. Transfers of property upon dissolution of the relationship are also tax free for legally married couples, but not for unmarrieds. Marriage can bestow a bevy of important benefits, including military or Social Security benefits, health care and nursing home coverage.