Last summer, Christine spotted a hickey on her then-15-year-old daughter’s neck. When she asked where it came from, her teen turned bright red. That’s when the girl confessed to dating a female classmate and came out to her mother as bisexual — joining a trend which, new data shows, is making Gen Z women the most openly bisexual group in society. "I really don’t care about her sexuality. I mean, my sister is gay." The mom told The Post. But secretly Christine couldn’t help wondering: Was her teenager — living in one of California’s most affluent cities and a freshman...