If the wife stand against you, you have slim chance to win....
BUT....
1) If you have evidence stating that his wife sleep with you willingly... your chance is higher..
2) Like a bro said.. it takes two to clap... so you can hold on this point firmly....
3) You can go find out what does the husband do and hows their family life..... if their family life is cold and distant.... you can say, the relationship between them is cold and distant ... and also because of the cold relationship, the wife is feeling lonely due to lack of family love, husband love and concern... so she find you to start a relationship... you are reluctant initially but you agreed when she insist that she will divoce or she don't love her husband anymore.... (BITE her back !!! )
4) If you managed to find out that their relationship is not that good or cold or near the edge of breaking and both of them did not put in any effort to save their marriage...... So you can question them the reason of sueing you... is it because of the compensation that they will get if they manage to win the law suit ? (BITE them BACK)
The above 4 points are just a simple suggestions from me.... maybe it sound naive or stupid... but it is only meant for suggestions...