Mitch's boyfriend is a character who does not formally appear in the movie. In fact, he is not even mentioned until the last scene of the movie, after Norman Babcock manages to save Blithe Hollow and put Aggie's spirit to rest.
Amid the rubble on the morning after the 300th-anniversary of the witch's curse, Courtney (who's been crushing on Mitch since first meeting him) suggests that she and Mitch go and see a movie together, preferably a non-horror movie (especially after what they and the others had just gone through). Mitch, who clearly had no clue that Courtney was romantically interested in him, seemed to think that Courtney's suggestion was purely platonic in nature and agrees to it, telling her, "You're gonna love my boyfriend. He's a total chick-flick nut!"
This is the only direct revelation of Mitch's private life that the audience receives, in which the only information is that he is currently in a homosexual romantic relationship. Mitch's exact sexual orientation (gay, bisexual, pansexual, etc.) is never revealed, but he must be gay at the very least.