Real feelings can develop on either or both sides. These SB are young and inexperienced with relationships. You might be the guy to give her 1st orgasm, or the 1st guy to really treat her well. Keeping things discreet can be a challenge and if you are in the relationship for a while, she is going to know your real name.
When things end, hopefully they end smoothly or else the SB can really fuck up the life of a married SD.