I had this issue also. I think some mechanics tighten them with an air gun to the point where I
had to replace the oil pan because they put a small , hair-line crack right next to the drain plug. You can also try using a rachet with a long bar for more torque to loosen the plug.

 

Once you get it off, make sure you replace it with a new OEM drain plug and crush washer. If you have a torque wrench, be sure to look up the torque setting in the owners manual and don't tighten the plug more than the setting calls for. For example, my 2000 EX-V6 calls for 29 lb/ft. If you overtighten, you will begin to strip threads.