People are generally understanding of the law with regard to equitable distribution in a divorce, including the exemptions for pre-marital property and gifts/inheritences. However, when two people never get married but live together, their rights are less clear. Fortunately (or unfortunately depending upon which side of the equation you are on), the courts have equitable powers to ensure fairness.
The initial, imperative question is whether a joint venture was created, despite property being titled in only one party's name. In general, a joint venture may be established in the absence of a written agreement, inferred from conduct of the parties. Generally speaking, some or all of the following elements are present in a joint venture: (A) A contribution by the parties of money, property, effort, knowledge, skill or other asset to a common undertaking; (B) A joint property interest in the subject matter of the venture; (C) A right of mutual control or management of the enterprise; (D) Expectation of profit, or the presence of "adventure," as it is sometimes called; (E) A right to participate in the profits; (F) Most usually, limitation of the objective to a single undertaking or ad hoc enterprise.
In the context of an unmarried couple, the factors that loom largest are the parties' joint contribution of money and effort toward the acquisition, upkeep, and improvement of the property; their venture's singular interest in the property; and the parties' expectation that if their relationship ended, they would receive an equitable share of their joint investment, regardless of formalities of title.
Once a joint venture is found to have existed, the court will make an equitable division of the joint venture's assets, which may include a forced sale or a forced buy-out.
If you have been engaged in a long term relationship without having gotten married, it is important that you speak with an experienced family law attorney to determine what rights you may have in the joint venture of your relationship.