An emotional affair generally starts innocently enough as a friendship.
Here's information about what an emotional affair is, how an emotional affair
differs from a platonic friendship, and why they are not healthy for marriages.

Through investing emotional energy and time with one another outside the marriage relationship, the former platonic friendship can begin to form a strong emotional bond which hurts the intimacy of the marriage relationship.

While there are those who believe that an emotional affair is harmless, most
marriage experts view an emotional affair as cheating without having a sexual
relationship.

Emotional affairs are often gateway affairs leading to full blown sexual infidelity.

"About half of such emotional involvements do eventually turn into full-blown
affairs, sex and all." Source: MSNBC

For most individuals, the most hurtful and painful consequences of an emotional
affair is the sense of being deceived, betrayed, and lied to.

If it looks like an affair, it's appearance is evil towards the marriage.

Biblical Reference:
"1 Thessalonians 5:22."