How do you get into terraria jungle temple without using key?

Answer 1

You need a pickaxe with a pickaxe power of at least 210 to remove Lihzarhd blocks (the kind of blocks that make up the temple). The only items with that much pickaxe power are the picksaw (a rare drop from the Golem boss, who is found in the temple), the luminite pickaxes (which require luminite to craft, which means you will have to defeat the Moon Lord, which you cannot do until after beating Golem), the laser drill (a drop from the Flying Saucer in the Martian Madness event, which only occurs after beating Golem), or the Drill Containment Unit (which also requires luminite to craft).

To sum up: unless you have a friend who has beaten Golem at least once and can give you one of the items that will let you mine Lihzarhd blocks, you have to get inside the Temple with the key at least once and beat Golem before you can get inside the temple without a key and without "cheating".

If cheating doesn't particularly bother you, check out these methods:

http://terraria.gamepedia.com/Guide:Skipping_Stones#Jungle_Temple

If the reason you want to get in without a key is that you're having trouble defeating Plantera, try mining out a large open space in the Underground Jungle and fighting Plantera there. DO NOT leave the Underground Jungle and enter any other biome after summoning Plantera, because Plantera becomes "enraged" and MUCH tougher to beat (its damage output doubles, and its defenses are doubled in the first form and quadrupled in the second form) if you do so.