how a girl can sue me for proposing

Whoever, intending to insult the modesty of any woman, utters any word, makes any sound or gesture, or exhibits any object, intending that such word or sound shall be heard, or that such gesture or object shall be seen, by such woman, or intrudes upon the privacy of such woman, shall be punished with simple imprisonment for a term which may extend to one year, or with fine, or with both. According to u/s 509 ipc

Element of intent is sine qua non for 509 IPC.  Proposing on other hand is a compliment; respect; and honour by the proposer.  It fact proposal elevates the modesty.  Our country has a tradition of proposing to the girl or their family.  There are examples when such proposals were made through Barbers, Pandit, parents or relatives.  By any stretch of imagination, it Is not an insult.  

The other side of the coin, civil, also connotes proposal as an offer.  It is up to the woman to accept or reject such offer.

Dear Mr. Ashwin Staff

You have jumped too high.  India does not have that bad state of affairs.  Even in #Me Too movement, the courts had been very cautious before issuing summons in such complaints.