Saw a recent AIO post where a woman who had a promiscuous past had recently decided to become celibate and wait until marriage, and was telling this to her date, which obviously made him upset.

Personally I believe it's her right to make such a decision, and even a step in the right direction changing her ways and upholding her self worth.

Though at the same time, she can't expect her dates/BF to not have sex with other women, and he doesn't have to inform her about it either. Why?

Because their relationship is not sexual, she made that very clear. Someone you are not in a sexual relationship with does not have the right to know about your sexual activities. So he can pursue sexual relationships with other women, while dating this woman who's waiting until marriage, and it's within his rights to not tell her or even lie to her about it, because it's none of her business.