I was kind of grasping for the right word, and later, while reading a book, came across it, the word is they (sometimes) seem scandalized by the mere idea, which somewhat ironically you use in your post. I'm not judging them either way, in fact, I want them to be scandalous, another interesting point is when men hear me talk like that, they seem mildly outraged too, like "she's not a good girl", but I don't want her to be. But, like I said, mostly I don't even know they have a boyfriend, so the pearl clutching seems misplaced.
We are not in 19th Century high society New York, or the Victorian Era, but I guess with social media, shaming is still a very important, as is monogamy, and the two are usually linked closely.