The obsession with being another's "first" is rooted in purity culture, which was pushed by religions and society for a long time. It's the same reason people are obsessive about losing virginity as males. Society pushes the notion that girls become 'true women' once their 'purity' is taken from them by a man.
Obviously, there's a huge amount of bigoted sentiment behind those ideas, and most decent people today do not agree with them. However, the notion still exists as a kind of subtext in our society. It's been ingrained in everything (classic storytelling, music, art, history, etc) for so long that it will take time for the underlying idea to fade away.