Not at all. Human beings aren’t actually meant to be monogamous from an evolutionary perspective. Human ancestors lived in large groups where males and females would likely pick a different mate every time. Monogamy is a very recent thing in human evolution, and is generally a cultural rule rather than an evolved behavior. So if your mind wanders to what could have been with a different partner, that’s a normal and natural thought process. However, if it leads to you seriously considering another partner, that’s a sign that there is some strife in your relationship that should either be addressed, or the relationship should end.

Nah
Thoughts are crazy and the mind needs to be freed (probably bc it can't be controlled)
It's your actions that defines you so as long as you don't do anything, you're good