Yeah, the marriage is probably the case; cheating/cuckoldry/cuckqueaning has much more negative connotations than the polyamory and open relationships that are standard in TiTS, which makes many (me and my Steele* included) uncomfortable with it. Still, romancing Bianca is a good thing for her for spoilery reasons: her sons and first husband were killed in a terrorist attack, and her second husband kidnapped and bimbofied her daughter, and both died in a UGC raid. She has a lot of trauma to work through after those events, which will be explored and resolved in her next expack.
*In fact, Isabel only accepts sex with Bianca because it was the latter's idea and she reassured her that it was okay. This is despite the fact that Isabel views manual and oral sex as proper sex and thus doesn't believe the reason behind said reassurance. Also it was quite early in her adventures, so she hadn't developed her normal sexual confidence yet.