I am looking for a diagnosis for the following behaviour.
Not sure if this is a sexual issue or just a psychological impairment?

Behaviour:
The person is displaying what seems like an attention seeking disorder. However, is engaged to be married, has one child, happy in the home, appears to be extremely happy with life in general.

Fiance is in custody for 3 months. Now she has started seeing someone behind his back. This is not the first time. Seems to occur when needing attention. But has a very tight knit family with plenty of support for her looking after her child. This has happened a few years ago. Seeming very happy and seeking attention from someone else. Also, not intending for a long term relationship and feeling very guilty after sexual encounters have taken place.

If anyone would like more info please email me, or if you could give me any insight as to what may cause this behaviour and how to go about treatment for it.