Any kids involved here???
If so, guest, why don't you ask the children of split homes where one parent left the other for another person how good they felt about the new relationship.
My daughter dislikes her step Mother to this day. She loves my Fiancee' though. Wonder why? Maybe because the man I am with did not split her parents up.
"Love" is not that giddy emotion you feel when with a certain someone else. The Japanese have it a lot more sane than we do. Their word for "love" is synonomous with "respect". Love is an ACTION, not a feeling. And to be the catalyst for splitting up a marriage is one of the least loving acts I can personally think of. If you "loved" this man, you would consider that.
Get the counseling that others recomended, and figure out why you would even contemplate the doing of something as potentially destructive and selfish as encouraging a relationship with someone who has already made a commitment.