I don't know about Seamus Finnigan, but:

" "Romantic?" Hermione said. "They're both boys!"

"Wow," Daphne said, sounding a little shocked. "You mean Muggles really do hate that? I thought that was just something the Death Eaters made up."

"No," said an older Slytherin girl Hermione didn't recognize, "it's true, they have to get married in secret, and if they're ever discovered, they get burned at the stake together. And if you're a girl who thinks it's romantic, they burn you too." " -From the beginning of Chapter 42

It would seem to imply that being gay is certainly accepted, so much so that the Death Eaters used the Muggles' homophobia as an argument against them.