I don't think anyone in the general public cares whether TA had premarital sex (I sure don't). But it's a really big deal with his Mormon community. In the Mormon belief system, premarital sex is a very big deal.
The issue is more about having a double life. His public image portrays the "worthy life" Mormon man, and his "success" as a motivational speaker and esteemed member of the community rides in large part on this image. But his actual lifestyle is contrary to the Mormon definition of "worthy".
So the defense will probably argue: If one lies or misrepresents himself in this respect, might he have other characteristics that were secret or hidden, as in being abusive? And if his life was "secret," no one else would really know...
And this IMO is probably where the defense is going with their self-defense claim; that although no one has come forward to say they witnessed these things, there was abuse, it was secret and part of an abuse cycle.
I can only speculate that this is a feature of an abuse syndrome which Alyce LaViolette, the DV expert due to testify for the defense, will talk about.