My girlfriend allows me to date other women. Why do so many people get upset about this?

We used to have an open relationship for many years and she has dated others in the past. She doesn't want to anymore and still allows me to. Every time I mention this, other women get pissed at me like I'm doing something wrong. I only mention this as to not give girls I'm interested false impressions that they're the only ones. I do this very early on but still, I get many that get upset at me and then get preachy. Should I just give up or is there some sort of app/community that is more accepting of this someone can point to? And yes, my girl knows of all my dates. She needs to approve of them before I can date them.
My girlfriend allows me to date other women. Why do so many people get upset about this?
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