Normally I agree, but it's something that they most likely agreed to
That doesn't mean it's GOOD. Extramarital affairs are pretty much always awful. Just because you agree to be a party in the affair doesn't make it okay. It's still really stupid, and highly selfish.
Bianca's husband is said to be a decent guy- he doesn't hurt or abuse her in any way, and seems to give her lots of freedom, nor has he ever hurt Varric in any way. It's WRONG for Bianca to cheat on him.
Varric is deeply in love with Bianca- or he's fooling himself into thinking he is. But he's got the short end of the stick here. He's basically a boy-toy for whenever Bianca wants him. Also- He shouldn't be involved with a married woman in the first place. He should have just left. It's WRONG for him to participate in this affair.
Now Bianca is the real idiot here. She's married to a presumably nice guy that she is cheating on with another man that's hopelessly in love with her that she just strings along. She is a bad person. She wants to have her cake and eat it too. Not to mention the fact that her stupid greed got Red Lyrium into the hands of CORYPHEUS, directly contributing to hundreds, if not thousands of deaths.
Basically Bianca is a sucky person, and Varric is a tool when it comes to her. Their relationship is one big dark hole going nowhere. Consensual or not, they shouldn't be together.