Why do people cut off their friendships when they get into a New Romantic relationship?

It just doesn’t make any sense whatsoever. For example, a girl I once knew told me we couldn’t be friends anymore because apparently her boyfriend “wouldn’t like it” but why should that matter? She says she’s doing it “out of respect” but I don’t really buy it since I don’t understand the reasoning of cutting one’s friendship for a man/woman who you don’t even know how things will pan out. Am I missing something?