Fred and George found out about Percy and Oliver because they found a Quidditch sweatshirt in his drawer and “No one in our family would ever like that team.” Ron found out from Harry who saw them snogging after Quidditch practice. It went something like “When did Percy and Oliver start uh you know.” Ron turned almost purple and his freckles nearly shot off.
Seamus and Dean actually didn’t see much of each other for a while after graduation. Seamus went to America to study pyrotechnics. Dean showed up in America after the Christmas card incident and met Seamus in a muggle pizza parlor. Dean didn’t realize they didn’t floo in America and frightened the pizza maker. All in all, they spent a long time laughing before they even started talking. It was a good thing they left the vicinity of the pizza place.