The Temple of Love is a room in the game that shows all the heart cubes you have collected thus far in the game.
In order to access it, you need to drain the water in the hub area you enter at the beginning of the game. Afterwards, you can enter a door that will take you to an island in the distance. The island is pyramid shaped with a single tree on top of it. The main thing this island houses is a door that requires all 64 cubes in the game to open. Once you have all the cubes, the door will open and you can enter the temple.
Once inside, you will need to do some vine-climbing the reach the top of the room. Once you are there, there will be a small fountain of pink liquid that seems to support all the heart cubes you have collected thus far. The heart cubes are suspending slightly higher than you can jump, so they are just there as an exhibit.
You can initiate a secret event in this room by taking the letter and number artifacts and rotating them left and right a certain number of times. This will cause the heart cubes to fall apart and fly upwards. Your game will also restart. When you load your save file again, you will still be in the heart room, but the fountain and heart cubes will be gone, most likely meaning that you deactivated the room somehow.