Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy:
Why lov'st thou that which thou receiv'st not gladly
Or else receiv'st with pleasure thine annoy?
If the true concord of well-tuned sounds
By unions married, do offend thine ear
They do but sweetly chide thee, who confounds
In singleness the parts that thou shouldst bear
The next  quatrain  states that, if the young man dislikes harmonious music, it’s because it reminds him that he is single and not procreating. The verb “confounds” means confuses or destroys. His singleness is therefore a rebuke to his potential for child-procreation. 
 There is a sexual innuendo in the phrase “the parts that thou shouldst bear”; referring to genitals and to bearing of children.
Sonnet 8