If the DNA had to get doubled during sexual reproduction, then each generation would have double the amount of DNA content as compared to the previous generation.(here double during sexual reproduction means what)

During sexual reproduction, Male gamete fuses with the female gamete. Both the gametes are with half set of chromosomes i.e. they have half the DNA amount. When both the gametes fuse, the DNA amount is restored back (Half amount from the male gamete and half from the female gamete). Therefore, in every generation, DNA amount is maintained and it does not double.