My only love sprung from my only hate!Too early seen unknown, and known too late!Prodigious birth of love it is to me,That I must love a loathed enemy.

This quote is saying that it is a strange and amazing thing to fall in love with someone you hate. It happened unexpectedly and much too soon, and it was too late when the speaker realized what had happened. Therefore, it is an incredible and unusual situation for the speaker to find themselves in.