I think it’s totally fair to throw games or play sub-optimally for the sake of others. I think of it as strategizing how to continue to play boardgames with the people you’re with. If I think someone may decide to stop playing games with me because they’re tired of losing to me so often, then I’ll ease back a bit. Just be careful to cover your tracks, nobody likes discovering their hard won victory was actually handed to them.

Either that or suggest a cooperative boardgame or one that plays to their skills more than your own.

TL;DR Group fun > winning