Dean and Seamus were totally a couple but Harry is just the least observant ever or he's so chill he just doesn't bring it up. You have to fight me to change my mind on this.
concept: several years after graduation, harry gets a letter from dean thomas asking if he’ll be his best man for his wedding- he, seamus, lavender, and parvati are having a double wedding. harry says yes, of course, he’s very flattered (though a bit surprised seamus won’t be best man, but he supposes they’ve got the double wedding thing going on). he’s a little embarrassed, though, because he’s not actually sure if dean is with parvati or lavender, but he figures he’ll just not put names on the wedding gifts and work it out when he gets there.
fast forward to the practice for the ceremony. harry shows up ready to best man the hell out of this, even though he still doesn’t know who the bride is. but oddly enough, the girls’ dresses don’t really match the tuxes? parvati is in traditional indian clothing with bright pinks and yellows, and lavender is wearing a light pink and white dress that complements it surprisingly well. dean and seamus are wearing contrasting white and black suits, respectively. harry comments on this oddity to dean, asking why they don’t match the brides.
“mate,” dean says, looking harry directly in the eyes and probably losing eight years of his life in that moment. “seamus and i have literally been dating since fifth year. every single one of us is fucking gay.”