How can I convince my friends there’s nothing wrong with being gay ?

Assuming your anonymously posted story is true, just by showing them that gay people (ie. you) are friendly, loving, accepting people, and that when you're in a relationship you and your mate share the same beautiful romantic love that any hetero couple does.  Beyond that, there's not much you can do to change prejudiced bigotry.

You can’t. All you can do is be the best person you can be. Either they will acknowledge it or they won’t. 

Think for your self. Have your own opinion. Always be ready to make new friends. Look in supportive ways of the gay community like gay community centers, charities and social groups. 

You cant other than by demonstrating there is no difference.

You may need a new circle of friends.

Tell them you still have the same interests you had when you became friends. Also assure them you will respect their boundaries and won't hit on them or anything 

you probably wont be able to