What is the process that is necessary for sexual reproduction?

Assuming you mean biologically, it would be Meiosis.

Meiosis is the term used for sexual reproduction. It essentially means that organisms that need Meiosis in order to reproduce need two of the same organism. In the process of meiosis, the cells divide differently than that in mitosis.

In mitosis, organisms asexually reproduce and do not need a partner as, essentially, the process is just organisms splitting and producing a parent cell and daughter cell.

Hope this was the answer you were looking for and that it helps!