Here, have something I wrote that I really should screencap to mire easily post:
>As the Omnitrix is supposed to bridge the gap between species and create bonds between them, maybe it has a hidden function of implanting the selected species' mannerisms, basic personality and social knowledge as well, with males and females getting different mannerisms installed accordingly. As Ben was born a human male, he isn't affected by the watch's personality data for males, and instead of acting like a stereotypical male acts like himself. This doesn't apply to him becoming a human female, and in this form the personality data would override his own and make him act like a stereotypical female.
>Ben wouldn't be too familiar with the gender function of the watch and not be too sure of how to reverse it, so he would be stuck in this form for long periods of time before he accidentally manages to make it turn him back. These periods of time would be made even longer due to Ben's new female personality understandably wanting to stay female. Unfortunately, this function of the Omnitrix can cause issues when changing genders of the user's original base species, due to it being so incredibly close to what the watch recognizes as being the "default" setting, and extended periods of use in this form not only begin to make the new personality slowly start overwriting his old one, but also have the watch start recognizing his female self as his new "default" setting and form.