>>714525682
There are generally three reasons for that headcanon.
The first are Aerith haters. Tribalism means that if you like, say, Tifa or Yuffie, then you want to bring the other heroines down. In the case of Aerith, saying she's a prostitute is an easy way to knock her down a peg.
The second are fedora tippers who overanalyze the game. She sells flowers for a gil each, which according to them should be impossible to live off of. So surely she must be doing something else to make money.
The third reason is partially related to the previous, but there are another branch of fedora tippers who think that Aerith being a prostitute makes the game more "mature" and "deep". These people don't necessarily hate Aerith, but rather they want to "elevate" the game.
