Question

How does sexual reproduction result in variation of diversity of characters in the offsprings

Answer 1

Meiosis is the process by which most eukaryotic organisms, those with cells having an organized nucleus, produces sex cells, the male and female gametes. Cells normally have two copies of each chromosome, one donated from each parent. This is the diploid chromosome condition and the paired chromosomes are called homologous. Meiosis transforms a diploid cell into four haploid granddaughter cells, each having a single copy of each chromosome. The process helps increase the genetic diversity of a species.

Answer 2

Sexual Reproduction involves two parents, thus it involves two different Chromosomes. The offsprings inherit a nuclear set with traits from both the parents. Hence variation arises.

Let us look at this way. 
Let's take Chromosome 19 (That's the chromosome which decides the colour of your hair.)
Your mom is Blonde and has a genotype of BB 
Your dad is a Brunet and has a genotype bb.
B is the dominant allele while 'b' is the recessive allele.
Let's make a Punnett square to see what your genotype.
refer (Punnett Square 1) 

So you see, You are more likely to be a Blond but since you have a mixed phenotype, it is possible that your future progeny may receive the recessive trait and be brunet. 
Suppose you marry a person of a similar mixed Phenotype, hence he/she is also Bb. 
(Refer Punnett Square 2)

Also, the Nuclear proteins,
Adenine, Guanine, Cytokinin and Thymine change their orders to form different gene codes. This also brings variation.