Is my marriage to a 15-year-old girl for pleasure, while I am 60 years old, considered unlawful and sinful for me, knowing that I was unaware of its unlawfulness and believed it to be permissible, and I have given the girl her due?

Details are necessary to determine the Shar’i ruling regarding what is meant by “Mut’ah”. If what is meant is a legitimate marriage with the intention of permissible enjoyment of the wife due to her young age, then there is no objection to that. Marriage is legally permissible for a minor if she is capable of intercourse, and the age difference is not a barrier. However, if what is meant is the well-known Mut’ah marriage, which is defined by a specific duration in the contract, then this is forbidden and invalid by consensus. There is also marriage with the intention of divorce, where the man intends to separate from the wife after a period without stipulating this in the contract. Its ruling is a matter of disagreement, but the majority holds it to be permissible and valid, and this is the fatwa given.