Legal Question in Criminal Law in California

No, unless it actually made him ineffective -- in which case,= the problem would be that he did his job poorly and not that you and he were having an affair. Effective representation won't be deemed ineffective just because the lawyer used questionable judgment in his personal life.

If you have a good argument that he really was ineffective, then evidence of the affair might help you prove it. To illustrate: Suppose the client's husband could have given great testimony, but the lawyer didn't use him as a witness for fear of being found out. If the lawyer claimed there was some other, valid reason why he didn't have the husband testify, the affair might discredit his argument. In other words, by itself having an affair with the client doesn't mean the lawyer was ineffective, but if he actually was ineffective then the affair could explain what happened.