Emotional affair is more common than sexual affair, but sexual affair is more talked about, more popular, and given more attention than emotional affair.
It is more difficult to notice emotional affair when it starts, unless you’re very careful, extremely observant, an empath, emotionally intelligent, or told by an onlooker.
Unlike sexual affair, emotional affair is quite tricky; it starts as emotional connection.
Emotional connection is not a bad thing per se; but the timing, the circumstance, and the person involved matter. One of these determines whether it is right or wrong.
• You may not be cheating sexually, but actually cheating emotionally.
• You could be a virgin or celibate, and still be in an emotional affair.
Some people may think they are being nice, transparent, friendly, and polite to a person; when they are in a relationship already.
It may start innocently, but eventually morph into an emotional affair – and this always affects relationships; because it means one of the parties in the relationship is not emotionally available to the other person anymore.
• Unfortunately, a person in a relationship could be so naïve, unsuspecting, carried away, or comfortable that they fail to realize that they are emotionally attached to a person (they are not supposed to be attached to).
• If you are not careful, you could be so close to a person’s partner/spouse that they become emotionally connected with you.
Like I said earlier, it can be very tricky!
Please know that there is a difference between “being extremely careful” and “shutting people out”; the line is kind of slim.
A clue: Always read between the lines and be extremely careful WHEN you are already in a serious relationship.
NB: Understanding this has helped so many people in marriage and relationship from having (or falling into the temptation of having) emotional affairs outside their marriage and relationship.
Some temptations come directly, some come indirectly; some people come into your life innocently, some come with an agenda. But, you should not use one to judge the other; you could be very wrong.
Again, it is very tricky. 
PS: Men are more prone to sexual affair, while women are more prone to emotional affair. This is not to justify or encourage cheating in any way.