it's shiro and adam's wedding. they're in cuba, because shiro's been living there for as while as a form of retirement and relaxation after the war (and for proximity to keith when he returns from or leaves for bom missions). keith and shiro have this whole thought out plan which everyone close to them (except lance) is in on: shiro throws the bouquet directly to lance, and after shiro and adam's farewell, when lance and keith are going back to the mcclains's place (keith lives with lance, and lance likes being around his family and stuff for the most part), everyone stops by at varadero beach.
keith drags a slightly confused lance out of the car and proposes to him under the moonlight with the sound of calm ocean waves in the background. lance is standing there, mouth agape, and he puts his bouquet down and fumbles a bit before taking out a ring box of his own. apparently he's been carrying it around as a promise to himself for a couple of months - keith didn't expect that nor did he account for that. everyone is quiet while lance proposes too. they cry, they hug, they kiss.
many hugs and cheers from the mcclains, team voltron, and adam and krolia ensue.