Is the phrase "Single until married" a reasonable term or a justification of deceitful and disloyal behavior?

I this question was inspired by the ongoing conversation online that I always see people justifying cheating by using this common phrase. have never been the person to jump on the bandwagon when it comes to that phrase because I think that it is used as a immature cop out excuse to cheat and be dishonest. Some people believe that unless you are legally tied to someone, then you are entitled to date multiple people with no obligation of commitment. But in my opinion if you are romantically involved with someone both parties are accountable for being honest with each other of knowing where the two of you stand in the relationship. Because I see a lot of commentary online about this topic and people use it as an excuse to cheat and we'll see after the fact that because they are not legally married, then they are "single". I personally do not trust anyone with this kind of mentality because it's very misleading to hide the fact that you do not want commitment.
Is the phrase "Single until married" a reasonable term or a justification of deceitful and disloyal behavior?
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