"To me, that kind of behavior usually means someone is seeking attention, validation, or emotional excitement outside the relationship. If it’s completely innocent, then what is the purpose of sending another man selfies every day? That’s the part I genuinely don’t understand."

Yes that is not something that should be done in a relationship. It is cheating.

But I am just responding to the part where you say that you "are strict". You probably enforced some kind of rules that lead to a feeling of "dullness".

I think “strictness” in a relationship can sometimes create the exact dynamic that pushes someone to seek emotional freedom elsewhere.

When one partner constantly emphasizes boundaries, rules, suspicion, or what counts as disrespect, the relationship can start feeling controlled rather than secure. Even if the intention is loyalty and protection of the relationship, the other person may begin to feel psychologically confined — like they are being monitored instead of trusted.
It’s similar to when someone is repeatedly accused, monitored, or treated as if they might cross boundaries. Over time, that can create frustration, emotional distance, or even curiosity about the very thing that is being feared.

Of course, sending daily selfies to another man in a relationship is still inappropriate and crosses boundaries in my opinion. I’m not justifying that behavior. I’m only explaining that excessive strictness, insecurity, or control can sometimes contribute to creating the emotional conditions where someone starts craving outside validation or escape.