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Oak and Agatha have fugged at least once in their lives.
Falkner/Janine
Erika molests Jasmine regularly.
Will/Karen
May/Brendan
Gabby/Ty
Brawly/Roxanne
Wallace=Sex
Dawn/Cynthia
Candice/Maylene
Flint and Volkner frequently double team chicks.
Juniper and Fennel 'experimented' in collage.
Nate/Yancy
Rosa/Curtis
Elsa/Skyla
Shauna/Protag
Sycamore=Sex (most notably Lysandre and Diantha)
Dexio/Sina
Viola/Grant
Korrina/Lucario
Kuiki/Burnet
Mallow/Kiawe
Molayne/Plumeria
Marnie/Protag
Honey/Mustard
These are the only guaranties I can think of, off the top of my head. Apart from other confirmed spouses.