Praise be to Allah.
Homosexuality is a major sin which deserves the most severe of punishments in this world and in the Hereafter. See Question no. 5177. But the person who does that cannot be described as a kaafir because of that, unless he thinks that it is permissible and claims that it is halaal – in which he is to be denounced as a kaafir. But simply doing it whilst admitting that it is haraam does not put a Muslim beyond the pale of Islam. He is still a Muslim, but he is exposed to the wrath of Allaah and is committing a major sin. It is not permissible for you to go ahead and marry him until he repents sincerely. Simply promising does not count as repentance. My advice to you is not to go ahead and marry him, rather you have to look for a righteous man who will be the means of bringing you happiness in this world and in the Hereafter. Homosexuality, just like other major sins, is open to repentance: whoever repents, Allaah will accept his repentance.