I don’t recall totally how they’re used in Shrek, but I’m pretty sure it’s from the “transphobic trap” story. When Shrek was made it was perfectly acceptable (maybe even encouraged) to make fun of trans people and to call them traps.
Sorry for the slur if it offends anyone, but I’m pretty sure that’s what this trope is.
I just don’t find males attractive. I don’t find male genitals attractive.