Question

Gametes must be produced by meiosis for sexual reproduction why?

Answer 1

two organisms of opposing sex contribute their haploid gametes to form a diploid zygote .The zygote undergoes meiosis immediately creating 4 haploid cell.these cells undergo meiosis to create the organisms

Answer 2

Meiosis process,If I go really simple,in which a cell with normal number of chromosomes that is 46 divides in such a way that the chromosomes are divided into two haploid cell,i.e,23 chromosomes in one cell and 23 in another.So up to here the 1st phase completes.
In the 2nd phase these Haploid cells with23 chromosomes divide by the method of mitosis.And hence gametes are produce.
When the gamates,male and female,fuse together they form the zygote ,which is the first cell of the body of the offspring,with the normal number of chromosomes/becomes diploid.
Hence it is necessary that the gamates must be produced by meiosis for sexual reproduction in order to gain haploid configuration (a cell with half the number of the actual number of chromosomes).