Does anyone have any recommendation on reading around the topic of sexual openness with romantic monogamy? Or any personal experiences with the same?

While it's not that I've never had casual sex, there has always been an emotional attachment of sorts. It's very rare that I would have had sex with someone where I didn't see (based on the limited info I had) a chance that we could potentially develop this into something more. And if sex can be something casual and fun between me (or a my partner) and another then how is it that it feels so intimate when it's between the two of us?

And what are the ethics with regards the third party? I have been the third in the past and always felt a bit odd when my hook-up casually mentions their partner as if it is no big deal. Knowing that instantly I had no future with them. My role was limited and dictated by another person who I had never even met etc. Or that they may be lying and maybe their partner has no idea.

I'm not necessarily looking to be convinced that sexual openness is right for me or wrong for me, or even that it can be split from romantic openness (not even sure these are the right terms) just looking to understand and develop my own stance.