I would like to know, if I have a girlfriend, is it forbidden? Why? Thank you.

For a man to take a girlfriend or mistress is forbidden and among the greatest abominations. It is a form of indecency (fahisha) that Islam has forbidden and prohibited. Allah has forbidden both men and women from taking khiddan (secret lovers/friends), as He Almighty says: "And do not approach immoralities - what is apparent of them and what is concealed," and regarding women: "chaste, not unlawful sexual partners or those who take secret lovers," and regarding men: "chaste, not unlawful sexual partners or those who take secret lovers." Islam forbids any relationship between the sexes outside the bounds of marriage because such relationships lead to great corruption and disobedience to Allah, in addition to unlawful seclusion (khalwa), as the Prophet, peace and blessings be upon him, said: "No man is alone with a woman except that a mahram is with her, for indeed the third of them is Satan."