NTA. That’s your son. He and his boyfriend are your family. Melanie and her parents are asking you to exclude your family because they are uncomfortable. Like others are saying, it’d be a running theme. Anytime they’re visiting, family holiday, birthdays, whatever, they’ll ask for his not to come because they’re “uncomfortable”.

Honestly to me, it just sounds like Melanie agrees with them and doesn’t want the boyfriend at your wedding either. Otherwise, if she saw you and your son as her family, she would’ve stuck up for him and his boyfriend. So not only are her parents making their choice, seems to me like she’s making hers.