Most Helpful Guy
About your update, it's not true. Nothing in this world is perfect. Regardless of who we end up with, we'd be lying to ourselves if we said someone is perfect. Not just because we all have our own ideal of perfection, but because that ideal is always changing.
I'll give you simple example - guys who leave the girls they're with, to be with another prettier girl. See here, the girl is only "prettier" in his eyes. And there's a common phenomenon that occurs with guys, that causes them to find people less and less pretty the more time they spend with them. This is called the "grass is always greener on the other side" effect, and it will make guys think other girls are prettier than the one he's with after a while.
This was just ONE example, and it's only related to looks and the effect it has on guys. However, there are TONS of more examples and situations that would cause a guy to feel like being with another girl. Maybe his girl doesn't do or say things that another girl does. Who knows? Does that mean he should keep on leaving the girl he's with every time he sees someone he could potentially be happy with? No. Or he's going to keep doing that for the rest of his life and never find someone to commit to.
It's normal to thing about being with other people sometimes. But, as long as you are already happy with the choices you made, it is unwise to pursue those thoughts. At the end of the day, it'll be your actions and decisions that matter. The fact that the decision was brought up in the first place should be irrelevant.