Considering the fact that Marriage is an institution one enters with the aim of unifying one's heart body and soul to another I personally believe Harman should not remarry at least not now. My reasons:
a) His heart and soul belong to Soumya alone, it would be foolish of him to marry another woman solely for the reason of bedding her. That in my opinion is unfair to his current wife (despite her sexuality Soumya is legally his wife), the new entrant and to Harman himself. It will be a battle of the sexes, considering Preeto will side with the new woman over Soumya and help her in making both Harman and Soumya's life miserable.
b) His marriage to another will only serve to push him more under the mountain of guilt he is already buried under. He believes Soumya to be his responsibility and knows very well that she loves him to the extent of madness. The new entrant will want sole authority over his mind heart and soul (cannot say body cuz that is the only portion of him she might get whole). At no point will Harman be able to accept the new woman as his love because his heart belongs to Soumya alone, that will cause friction not only in his marriage but also in the lives of his family members. He will be inflicted with emotional trauma and may end up hurting himself and others in the process.
c) Even when Harman will deliver Soumya to Nimmi Preeto and the Kinner group will make her life hell. To Soumya Harman is the only being able to protect her from the world and since Harman loves her too he will be there by her side come what may which will only increase their problems. How will the new wife react to this? Will she not want a normal life with her husband? Will she not want to have a peaceful wedded life with Harman? And if in all this a child is thrown in the picture then what? Things are going to get more complicated.
Harman needs to take Soumya to a long delayed doctor's visit get her checked up, discuss his options with the doctor trust me science can perform wonders today and there is always the adoption route.