Performing sexual services for remuneration is generally considered immoral, and a contract in which one party undertakes to provide a sexual service and the other party undertakes to pay remuneration for that service is usually considered invalid. Consequently, the remuneration paid for the service should be reimbursed under Art. 405 in conjunction with art. 410 § 2 of the Civil Code. A demand for a refund of remuneration for a sexual service may raise doubts as to its compliance with the principles of social coexistence. The situation might be analysed in the light of the Civil Code on two grounds, i.e. Art. 411 p. 2 and Art. 5 of the Civil Code. The study presents the rules for the application of both provisions, the relationship between them, and the possibility of applying them to the situation of a demand for reimbursement of remuneration for a sexual service. The analysis shows that Art. 411 p. 2 of the Civil Code, which allows the recipient to retain the benefit if it complies with the principles of social coexistence, should not constitute a legal basis for excluding the return of undue benefits performed in the course of a legal act that was incompatible with the principles of social coexistence. In such cases the exclusion of reimbursement of an undue benefit may be based on Art. 5 of the Civil Code.