This is the ambivalent cousin of slut shaming. Slut pitying occurs when people assume that a slut is only a slut because of insecurity, conforming to sexist standards, daddy issues, and the like. Slut pity assumes that a slut isn't a slut out of personal choice, personal discovery, or self-empowerment. Slut pity is generally a knee-jerk reaction and often comes from good intentions, though sometimes not.
Person 1: Trisha sleeps around a lot, she must be insecure.
Person 2: That's slut pity, which is quite judgmental. Maybe she just likes having a good time and is totally fine with herself.