Lack of knowledge of the marriage between the plaintiff and his/her spouse is a defense to an alienation of affection claim.  In contrast,    if you engage in sexual intercourse with someone who is married, whether you were aware of the marital status or not, a claim of criminal conversation may be filed against you.
There are a few additional defenses to claims of alienation of affection and criminal conversation which we would be happy to discuss with you when evaluating your case.