From fairest creatures we desire increase,
That thereby beauty's rose might never die,
But as the riper should by time decease,
His tender heir might bear his memory:
But thou contracted to thine own bright eyes,
The rose represents  ideal beauty in its prime . By reproducing, it could, in a sense, become immortal. 
 In Chaucer’s  ‘The Wife of Bath’s Prologue’ , the rose is associated with female genitalia; 
 For if I wolde selle my bele chose, I koude walke as fressh as is a rose…" 
 However, of the 13 uses of “rose” in the  Sonnets , Shakespeare uses 11 of them in the sonnets addressed to the young man: Sonnets 1,  35 ,  54 ,  67 ,  95 ,  98 ,  99 , and  109 . The last two, in  Sonnet 130 , are used specifically to point out what the dark lady is  not : 
 I have seen roses damask’d, red and white, But no such roses see I in her cheeks; 

Sonnet 1