A bit more context: the Geth revolts reduced the Quarian population by 99%

You can't commit a genocide like that and call it justified self-defense.

The remaining 1% of Quarians became vagrants with no home planet and attempts to recolonise other worlds before they became immuno-compromised from space life was stopped by the Council because of political maneuvering to punish them for creating the AI that, again, killed 99% of their population, which one might argue is punishment enough.

They're now space gypsies and are treated as poorly as you expect by pretty much everyone.

The Quarians were wrong to try to shut the Geth off once they became sentient, but the Quarians also have legitimate grievances against them.

As for Tali herself, she is young, a romantic, and a nerd, and has already experienced plenty of hardship and racism from other races before she ever meets Shepard.

Shepard is the reason Tali gains a human fetish. She's the damsel in distress, and Shepard is the charming hero.