If Dean fought all proper instead of playing dirty, Seamus would have won the first round of chugging drinks. Or so he said. But he didn't challenge Dean to another go because Seamus was a lot of talk, even when it came to drinking games. That wasn't to say he couldn't drink Neville or someone under the table, but Dean was bigger than him.
"People...that have had sex with Veelas. Like Fleur's dad. Shut it, don't try turning this around on me because you're gay and don't find female Veelas attractive, you great big wanker." Seamus might make a lot of jokes, but he didn't mean half the things he accused Dean of. Their friendship would have been in trouble if he did.
Spluttering in indignition, he waved his mug of beer - carefully, so not to spill a precious droplet - at Dean and said, "I'm always right. Your tendency to lie about these things warrants a visit to St. Mungo's psychiatric ward, I think."