I wanted to at least have a bro -option to tell Bianca that she shouldn´t write to Varric anymore, and let him live his life. Her being married and all. In a very snide way too. No such luck.
Given the setting and from what we know of the cut-throat nature of the Merchant's Guild, she is likely in a marriage of convenience meant to advance two families fortunes and probably has no feelings for the guy at all.
In the setting and in real life when given such circumstances, it was quite common for them to take on lovers aside from their respective spouse. Men would take mistresses and so on. Anora will mention to a warden romancing Alistair that Cailan had mistresses, Alistair can take the Warden on as a mistress if he marries Anora, and in my current playthrough, Leliana is a mistress to King Cousland and was announced as such at Celene's ball.
I'm all for honoring marriage and being true to your spouse, but I wasn't at all surprised about her dalliance with Varric considering the nature of the setting.