In a divorced couple, should the custodial parent be able to deny parenting time with non custodial parent, if said parent starts dating?

It's a complicated question. When I was a child my mom refused to let me see my dad because he started dating someone. Basically effectively cutting him from my life. I see that as incredibly unfair, but I also see this as a move that most if not all single mothers pull. It's not nearly as common with single dad's. Why is it OK for mom to say "if you start dating her you will. Never see your kid again"?
In a divorced couple, should the custodial parent be able to deny parenting time with non custodial parent, if said parent starts dating?
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