Don't say it just an anti on One Halyu started it to make shippers upset, let face the fact that this was buzzing around on Korean sites recently too. The broke-up rumor were surfaced among Korean sites (not fandoms) much around last month till now, if you guys often did research about YoonGi you might read about it, but it was nothing but just as questioning, and they were not antis at all.
But... even if this were YoonGi and the items were true, I don't think anyone has right to judge them, neither Yoona nor Lee Seunggi, I'm almost considered a Lee Seunggi's fan though. A star's love or parting should just be seen as God's fate in the eyes of anyone else outside since it's their life nor us.
Calm down, don't against anything we don't want to believe to happen strongly. No one know what will happen tomorrow so that even if the things we do not want to happen to become real, we would take it - not-easily or easier - but at leats less difficult - for the less shake less hurt of no one else but yourselves. Not just as a fan's life but as a real life.