I have to agree/disagree with Mikolas.
Well in a biblical standpoint its said " If a man even thinks lustfully towards another woman, he has committed adultery. Aka .. thats bad

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In a Non-biblical standpoint. Think of it this way.
Would you Chat with this person in front of your wife/husband/significant other?
would you look at pics of this person in front of your wife/husband/significant other?
would you call this person in front of your wife/husband/significant other?
If you said NO, to any, if not ALL of these questions then its a BAD idea.
You may not physically be cheating right then and there, but it can lead to it.
And it also causes alot of secrets between you and your significant other.
Which leads to mistrust, which leads to.. just bad things.
If ya wouldnt do it in front of your significant other, then its a BAD thing to do.
That also works with like... .EVERYTHING lol. Hope ive been helpful