The ugly act of liwat is one of the greatest sins which the Quran introduces as the sin the people of Prophet Lut (pbuh) that brought about their punishment and annihilation from God. This sin entails two types of outcomes; one is taklifi (has to do with our obligations to Allah), meaning that one who commits it has committed a great sin and great penalties and punishments have been specified for it, one being execution in a special manner. But if one sincerely repents from such a sin before it is proved in court, it is clear that Allah (swt) will forgive it.
The other group of outcomes are wad’iyy ones: There are some inevitable outcomes when it comes to this sin (such as the ruling that states that one can never get married to the daughter, sister nor mother of the person one has committed liwat with); outcomes that will not be lifted and undone even if one truly repents and is forgiven for the sin.
Imam Khomeini says in this regard: “The mother, sister and daughter of the person one has committed liwat with are haram on the one who committed it, even if they two weren’t baligh (pubescent).
Some of the maraje’ believe that this ruling only takes effect when the one committing it is baligh, but if the one liwat is being done to isn't baligh, then the ruling doesn’t apply. But there is no difference of opinion in the case of the one committing the sin being baligh; in this case the consequent ruling will definitely apply.
For further information, see:
1- What is liwat, Question 2829 (website: 3061).
2- The consequent rulings of liwat, Question 1647 (website: 1657).