Accidentally plugged in earphone jack into USB port! So I wanted to plug in my earphone jack into my Mac but I was not seeing so I accidentally and gently without that much force I put in my earphone jack into my  Mac left usb port but it did not touch the pins. So should I worry that I have damaged my Mac?
I wouldn't worry too much, the only two pins on a USB port that might have an ill effect if you short them together are the ground and power pins but they're located at either end of the connector so it's unlikely that would have happened with an earphone jack.

Additionally USB ports have over-current protection which is why from time to time you might see a message similar to "USB device was drawing too much power - USB devices have been disabled". In that case a reboot should re-enable the port. So unless you've physically damaged the connector, which it doesn't sound like you have, I don't see you'll have a problem.

Just to add a reference regarding shorting of pins from a publicly available standard, the Universal Serial Bus Revision 2.0 specification, sections 7.1.1 and 7.2.1.2.1:

Short Circuit Withstand


  A USB transceiver is required to withstand a continuous short circuit of D+ and/or D- to VBUS, GND, other data line, or the cable shield at the connector, for a minimum of 24 hours without degradation. It is recommended that transceivers be designed so as to withstand such short circuits indefinitely. The device must not be damaged under this short circuit condition when transmitting 50% of the time and receiving 50% of the time (in all supported speeds)...


Over-current Protection


  The host and all self-powered hubs must implement over-current protection for safety reasons, and the hub must have a way to detect the over-current condition and report it to the USB software. Should the aggregate current drawn by a gang of downstream facing ports exceed a preset value, the over-current protection circuit removesor reduces power from all affected downstream facing ports...