does Sexual reproduction relies on meiosis instead of mitosis because only meiosis produces diploid sex cells.

Yes, meiosis is strictly dealing with your gonads which are your gametes (testes/ovaries). It would not produce a diploid sex cell but rather 4 totally genetically different haploid cells. It can best be seen as 2n becomes n. In mitosis you can have 1n to 1n or 2n to 2n because in mitosis you have 2 identical daughter cells exactly like the parent cell.