It didn't become a regular thing.
They met a few times a week, most of the time at Ron's ruddy apartment above the shop of the Wizard Wheezes. Ron was helping his brother George with the shop, did home office work for the Auror Department and took care of his kids, Rose and Hugo.
Seamus liked being there. He liked Ron's children and it felt good to have someone to talk about the stuff he couldn't tell anyone else, to have someone he could trust to keep his secrets. Even if Ron weren't gay, Ron wouldn't tell...that's not how Ron was. Ron's an idiot, but he's also a loyal friend.
It was comfortable in Ron's tiny apartment. Sometimes Seamus was just visiting to play with children or they sat at the scratched kitchen table doing their work, exchanging a few words from time to time. Sometimes they listened to the game in the radio and sometimes, after the kids are asleep, they had sex.
It wasn't that there was anything romantic between them. It was just fun for them and convenient. None of them wanted to risk being found out so finding an outsider that they could trusr enough would have been difficult.
Ron's handsome enough. Seamus didn't really care for red hair and he had enough freckles of his own, but he was tall and lean and had angles instead of soft feminine curves. Ron felt a lot better than birds ever had. It was easy too, because Seamus knew, that Ron's not that invested in him emotionally.
Ron was so loyal and he tried so feckin' hard to not show how much he cared for Harry, that it was painful to watch. Sometimes Seamus asked himself if this is how Ron thought about him and Dean too. Seeing Dean being happy with his stupid Muggle wife is so painful that he rarely visited him.
They floated through their life, Ron's children grew older, the boyish softness melted from Ron's face and nothing significant happens. It was comfortable, but a bit bland and after a few beers Seamus sometimes thought, that he didn't deserve more than that.
Perhaps it was God's payback because he hadn't been to a confession or church for years and had been lying to all the people he held dear. Though Seamus suspected that if there was a God, they didn't really care for tiny sins like his. If he were God he wouldn't care.
It was fine. He was happy enough and as long as it meant to keep his friendship with Dean and the relationship with parents intact, he didn't mind being a lying bastard.
He was fine.
Ron was not.
He could see it every time Ron came home from the Auror Headquarter or met Harry at the Burrow. He was so feckin' jealous it was slowly rotting him from the inside. Ron came home, with hollow eyes and his lips frozen in a helpless grin. It's evenings like this, when they got way to drunk and had bad sex together that left bruises on his hips for week.
It was ugly when Ron was meeting with Hermione. He's missing her as friend and noone can ever replace her. Seamus still doesn't get the relationship between Ron and Hermione. They were hurting each other and fought ferociously like wounded animals. Hermione was brilliant in making it sting, making it cut deep. Telling Ron how much of a failure he was with his dead-end career, living from day to day without a major plan except keeping Rose and Hugo happy.
It wasn't one-sided though. Ron could do damage to no end with telling her how much of a bad mother she was. That she wasn't there when Hugo made his first steps or Rose learned how to read. That the one caring for them when they're sick was him and not her. He was the one who taught them how to bind their shoe laces, how brush their teeth and hold a knife. While Ron were a parent for them every day, Hermione's mothering is compressed to an afternoon once a month.
Ron didn't tell Hermione that Seamus was there to fill the gaps. That Seamus was there to read bedtime stories, scold them and heal scraped knees. It was natural, with him being around so much and Ron and him being each other's gap fillers in such a comfortable way. It felt nice when the children smiled and cheered when he was entering the apartment. Even he didn't feel like Ron was his partner and he's not in love with him, he still felt like a parent to Hugo and Rose.
When Ron made an important decisions for the children, he stopped asking Hermione, but he always wanted to know Seamus' opinion. It only felt natural that Seamus moved in the room that had been an unused home office...Ron prefered working at the kitchen table.
May they weren't a couple and they weren't in love, but with Ron and his children it felt like they were a family.
It worked great when there was no one else around. They were a good team. Life was comfortable and their days went smooth and ended with beer. It was a bit uncomfortable when George was there, staring at them out of his sad eyes, like he knew what was going on. He never said something though. Probably he didn't even care.
It was a lot more annoying when Ginny and Harry came to visit though. It was difficult to ignore that Harry's jealous. It wa like a flashback to their 4th year, with Seamus being the gap filler and Harry staring at him like he's the source of all evil, as Ron's attention was shifting away from him. As far as Seamus knew, Harry was quite happy with Ginny. He was just greedy. He wanted Ron's full attention without giving it back. He wanted Ginny to be his wife and he wanted Ron to mould his feckin' life around him and his perfect little family and the sad thing was that Ron really tried.
Sometimes Seamus was very angry after such a visit, with Ron staring into his fifth whiskey, like it could answer why his life hurts so much.
They didn't talk about this. Seamus knew how it was. Ron didn't need to say it loud for him to understand. It was the same after Dean's rare visits, just with reversed roles. The only thing he could do was just sitting there, try being a good mate and making sure Ron got to bed without hurting himself after he drunk himself into stupor.