How can a diode parallel to the source can't take the source value but take about 0.7V?
In an ideal circuit with a ideal voltage source with a perfectly constant voltage output and no output resistance, and ideal diode with no resistance and a constant voltage drop, there is no answer. It is a contradiction, which is why it is ideal and doesn't actually exist. It is basically unstoppable force (the voltage source) versus immovable object (the diode voltage drop). In general, this is why ideal things are ideal. They are simple to work with, but there's a logical contradiction lurking in there somewhere that doesn't allow it to actually exist.
But in reality:

the voltage source has output resistance which causes the voltage
output to decrease as the current supplied increases
the diode's voltage does change very slightly as current increases
as the current running through it increases

This means that as the current increases the source voltage output and the diode voltage drop approach each other. However, good voltage sources have low output resistance, and diodes take a lot of current to change their forward voltage drop by a little bit  so the practical result is usually that the voltage source has to output so much current that it explodes before the equilibrium can be reached.