Here are my thoughts:
You feel your sexuality is weakening. For proving the contrary, you make a romance with another person.
By chance - it happens that you may cheat on your partners without wanting it. It's like Bruce Willis - not in the right time and not in the right place
Relationship checking - if your partners find out about your second love, your partnership will either break down or become stronger
Marriage crisis - this is an almost unavoidable problem.
Searching for a better "option" - when you achieved many things while your partner didn't, you may want to find another person who would meet your requirements
Curiosity - this appears when one of the partners was not fully satisfied with something by the other partner.
For satisfying the curiosity, you need another person
Desire for gaining experience - if a person had very few partners before marriage, they will want diversity and will look for another person.
Revenge
After an inside family conflict, one of the partner can take their revenge by finding another person.
"I believe in making the world safe for our children, but not our children's children, because I don't think children should be having sex."
