A married man you had a relationship with "cheated" on you. The trauma was so difficult to deal with you avoid dating singles, even though it was a married man that cheated. Now, you are an enabler to those who do what terrifies you because you feel safer. Yet, you want nothing more than to marry a man, settle down, and have lots of children.

Appears to me to be a trust issue. You cannot trust a single person. So, you do what you are not proud of with people you have less reason to trust than many singles.

I wish you well in therapy.