Doesn't this mean that multiple private keys exist that can decrypt the ciphertext,
Well, yes. The relationship that a public and private key must have is
$$e \cdot d \equiv 1 \pmod{ \text{lcm}(p-1,q-1) }$$
Because of this relationship, then if we take a valid private key $d$ and add $\text{lcm}(p-1,q-1)$ to it (or a multiple of that), we'll come up with another valid private key.
In the example you have, $\text{lcm}(p-1,q-1) = \text{lcm}(1,6) = 6$, so adding 6 to the private key gives you another valid key. And, if you look at your examples, you'll see that all your private keys differ by 6.
making it less secure?
Well, no. For one, for realistic sized RSA modulus (rather than the toy example you have), $\text{lcm}(p-1, q-1)$ is huge (about the same size as the modulus); we don't have to worry about someone stumbling on a private key by randomly guessing them.
In fact, in this example, the public key itself can even be used to decrypt the cipher text.
Yes, that's another artifact of the tiny modulus. For real RSA modulus, this doesn't happen.. In fact, if we select a small public exponent $e$ [1], which is common practice, this cannot happen, as $d$ must be at least $\text{lcm}(p-1, q-1) / e \ge (p-1)/e$, which is for the primes we actually use, is quite large.
[1]: I'm assuming that you aren't silly enough to use $e=1$...
