There is two thing here , the H-game favoring male PoV and excepcting female non M-C to be okay with going from monogamous to polygamous relationship, because it is the male fantasy , or one of them. It's the Hentai/porn-world reality .
Second the fact that indeed event seem to put Rowan with an impossible choice ,join evil to protect his loved one , since I didn't yet play this part I can't really say anything about it but if its like Pseudonymous pervert said it's cheating yes but NTR? I don't think so , it's all about how Rowan act about it , is he a douche or repenting.
On the Other hand
@Pseudonymous Pervert I don't think a moralizing Alexia is that bad , it's show she has backbone , of course if she cheated herself its a bit hypocrite but than again it's heavily implied that she is suffering from Stockholm syndrome . It make her a tad more real and show that she is indeed attached to her vow despite the circumstance , there is depth to her persona . Then again in my play-through I intend for her to stay faithful .