Actually, he wasn't cheating. He was forced into a political marriage when he already had a relationship. She HOPED that he was bisexual with the Viscount, and when she found out that she was rightβshe (and her maid) died of happiness and fujoshi drool. So, basically, poly relationship. The wedding night was rather sketchy on the consent, but the BL appears fully consensual. In fact, I'd say that she *probably* didn't want to have sex with him at all, and would much rather *watch* him and the Viscount make love.
If you don't know what a poly relationship is, well basically, it means that they have multiple confirmed fixed partners. This isn't an "open" relationship. He doesn't appear to be doing it with *lots* of guys, but just with her and the Viscount (which, BTW, she HIGHLY approves of! 2πππ)