在中國大陸法律體系下，所謂的「同居關係」主要可以分為兩種情況。第一種是已婚者與他人同居。如果一方已經合法結婚，卻仍然以夫妻的名義與另一人共同生活，這樣的情形會被視為「事實婚姻」，並且依據《中華人民共和國刑法》的相關規定，構成重婚罪。這不僅涉及道德層面的問題，更觸及刑事責任，會面臨法律的嚴厲處罰。

第二種同居關係則發生在雙方均為未婚、處於單身狀態的情況下。法律並沒有明文禁止這樣的同居行為，但同居並不具備合法婚姻的效力，因此不受婚姻法的保護。換言之，雙方在法律上並不是夫妻，沒有夫妻之間的權利與義務。例如，他們不能以夫妻身份向對方主張財產分配、子女撫養權或是繼承權。這樣的關係更多被視為一種事實上的生活安排，而非法律承認的家庭制度。

因此，在大陸的法律觀點中，同居本身並非一概違法，而是要根據雙方是否已有婚姻關係來區分。如果一方已有婚姻而與他人同居，則構成重婚罪，屬於刑事違法；若雙方均是單身，則法律不禁止，但也不提供婚姻上的保障，權益需要雙方自行協商。

In the legal framework of mainland China, the concept of “cohabitation” can generally be divided into two categories. The first type is when a married person lives together with someone else. If a man or woman who is already legally married cohabits with another individual in the name of husband and wife, this is considered a de facto marriage and constitutes the crime of bigamy under the Criminal Law of the People’s Republic of China. Such behavior not only raises moral concerns but also carries criminal liability and will be subject to strict legal punishment.

The second type of cohabitation occurs when both parties are unmarried and single. The law does not explicitly prohibit this arrangement, but cohabitation does not carry the legal effect of a valid marriage, and therefore it is not protected by marriage law. In other words, the couple is not regarded as husband and wife in the eyes of the law and does not share the legal rights and obligations of a married couple. For example, they cannot claim property division, child custody, or inheritance rights as if they were spouses. This type of relationship is generally regarded as a practical living arrangement rather than a legally recognized family structure.

Thus, from the perspective of mainland Chinese law, cohabitation is not inherently illegal, but its legal implications depend on whether one of the parties is already married. If one party is married and cohabits with another person, it constitutes bigamy and is a criminal offense. If both parties are single, the law does not forbid it, but neither does it provide marital protection, leaving issues of rights and responsibilities to be resolved privately between the parties.