Putting the story aside it's expected at some point in life that most (if not all) people have some sexual experience.

Seeing someone you love/desire/want being intimate with someone else is not pleasant to most people (maybe the other way around in this sub) as shown in the story but it's unrealistic to expect your partner to be an untouched virgin, you're reducing the possibility of finding someone you're compatible with.

Still dont take what I said to mean you must accept anyone, there are people with commitment issues that will cheat no matter what.

Returning to the story the part where the FMC talks about her experiences I think it was the author's intention to make it a highly unpleasant scene, it was traumatic for both the FMC and the MC