Assalaamu alaykum. As far as I know, regarding things that have been unanimously agreed upon to be haram as halal is Kufr (disbelief). What is the ruling on someone who does not regard a haram thing as halal but boasts about doing it in order to become reputed in his circle? For example, someone boasts about how he flirts with girls and so on; is that also disbelief, or is it sinful?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

A Muslim is not declared a Kaafir (disbeliever) merely due to committing a sin unless he persistently claims that it is not a sin, as has been underlined by the scholars and was previously explained in fatwa 258232.

Flirting with girls is an evil action and a means to Fitnah (temptation). He himself may fall in temptations or lead those girls to it. The sin is graver when he openly boasts of it, because the Muslim is enjoined to conceal his sin and repent to Allaah of it in secret. Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said, "Every one of my followers will be forgiven except those who commit sins openly or disclose them to others. An example is that of a man who commits a sin, which Allaah screens, at night, and then in the morning he says (to people), 'I committed such and such sin last night,' while Allaah had concealed it during the night, but in the morning he tears up the screen that Allaah concealed him with.'" [Al-Bukhari and Muslim]

Ibn Hajar wrote:

"Ibn Battaal said, 'Openly flaunting one’s sins implies disrespect towards Allaah and His Messenger and the righteous believers, and it is a kind of stubborn provocation. There is no doubt that concealing one's sins protects him from disrespect because sins bring about humiliation to the people who commit them, such as the Hadd punishment (corporal punishment prescribed for certain crimes) or the Ta‘zeer penalty (disciplinary punishment) if no Hadd is prescribed. If it is something that violates the rights of Allaah, then He is the Most Generous and His Mercy precedes His Wrath. Hence, if Allaah, The Exalted, conceals it in this worldly life, He will not expose it in the Hereafter, but the one who flaunts it openly loses all of these privileges.'" [Fat-h Al-Baari]

Allaah knows best.