بسم الله الرحمن الرحيم
We begin with Allah’s blessed name, we praise him and we glorify him, seek his forgiveness and ask him to guide us. Whoever Allah guides, None can lead astray and whoever he misguides, None can guide. There is no power and no strength except from Allah, The most high, the Most great, the most powerful. We bear witness that there is no one worthy of worship but Allah Alone, and we bear witness that Prophet Muhammad (pbuh) is His slave-servant and the seal of His Messengers. We pray for peace and blessings on all the noble messengers and in particular on the last of them all “the blessed prophet Mohammad (pbuh)”
Dear brother, one should know that it is absolutely haram to have sexual intercourse with ones wife while she is experiencing her menses. If one in ones ignorance has transgressed any of the limit of Allah then the first and the foremost one should do is to seek sincere repentance of Allah for ones transgression.
Dear brother, every woman knows her regular days and the period of menstruation; and if out of ignorance or forgetfulness a women had sexual intercourse with her husband just at the beginning of her menses, then the couple should turn unto Allah in ‘taubah’ and seek sincere repentance for their transgression.
Your Statement: Are they gunahgar?
Yes they are sinners, as they have gone against the command of Allah, but they should not despair from the mercy of Allah for Allah says in the glorious Quran:
(O Prophet) say: “O My servants who have wronged their own souls….Do not despair of Allah’s Mercy! Surely, Allah forgives all sins. He indeed is the All Forgiving, All Merciful. Return to your Lord and submit to Him before the scourge overtakes you; for then you may get no help from anywhere.” Quran [39:53-54]
Narrated by Abu Sa'id Allah's Messenger (saws) said that the Shaytaan said, "By Thy Might, my Lord, I shall continue to lead Thy servants astray as long as their spirits are in their bodies!" The Lord, Who is Great and Glorious, replied, "By My Might, Glory and Exalted Station, I shall continue to pardon them as long as they ask My forgiveness!" Al-Tirmidhi 
Your Statement: And please tell what is the kafara for that..?
Narrated Abdullah ibn Abbas: The Prophet (peace_be_upon_him) said about a person who had intercourse with his wife while she was menstruating: He must give one dinar or half a dinar in alms.Abu Dawood [Book:1, Hadith:264]
Thus the kaffara for such a sin is to first turn to Allah for repentance and in addition to that one should give one dinar or half a dinar in alms. And Allah alone knows the best.
I ask Allah to make this a sincere effort, seeking his pleasure, and I ask him to grant us refuge in him from the evils within ourselves, and that in our deeds. I ask him to grant us success in achieving whatever pleases him; And May Allah Shower His blessings and mercy upon our beloved Prophet Muhammad (pbuh), his family and his Companions and on all those who follow him until the final hour.Ask Your Question