No, not against the TOS, and not abuse. If you use the gender of your avatar (male or female) to dishonest ends, that also isn't against the TOS, but is definately not okay.
The really amazing thing, to my mind, is that since we can be anything or any gender, our real person shows through more. That isn't to say that if you use a female av, you are either affeminate or a perv, there are sooooo many other reasons (the ability to find clothing is a good one), but it does say something about you. Don't worry, it's probably good