Depends how you hamster it:
Redpill rogue: she is unhappy because her boyfriend/husban is lame as fuck - fucking her you are "helping her being happy" in a way, and you don't give a shit about some beta cuck.
Redpill lawful: you are a man, bros before hoes - don't cuck your own people, we should stay strong together.
To me, fucking a girl in relationship is OK. She wanna cheat she will - I gladly take that burden. But after I'm done with her I would probably tell the dude that she is a whore and also send him some redpill material to study.
Redpill rogue: she is unhappy because her boyfriend/husban is lame as fuck - fucking her you are "helping her being happy" in a way, and you don't give a shit about some beta cuck.
Redpill lawful: you are a man, bros before hoes - don't cuck your own people, we should stay strong together.
To me, fucking a girl in relationship is OK. She wanna cheat she will - I gladly take that burden. But after I'm done with her I would probably tell the dude that she is a whore and also send him some redpill material to study.