Everyone thinks he is gay, but I know for a fact that he is because I've met his wife.

Straight

Explanation:

A gay person is one that is considered to have feelings of a sexual nature or attraction for someone of the same sex.

A straight person is considered typically to be someone who is heterosexual, i.e attraction to the opposite sex.

So here, the man in question was thought to be gay but was disproved because the writer has already met his wife.

'not' is the missing word

Explanation: having a wife negates the fact that he is gay so he is 'not' gay

the first step is to convey the appropriate emotions.