I can see both sides of this issue. If you plan to have a casual relationship where you date, hold hands, and do not see it ever moving beyond kissing then you could consider remaining stealth; HOWEVER, you should also consider the consequences for the other person if someone, somehow outed you to them or to the general population. Beyond just hurting the relationship you could also cause your girlfriend a lot of pain and confusion regarding her own sexuality. You and I know that dating a trans guy does not make you attracted to women, but your average Joe has a hard time understanding this. The teenaged years are a very important time in a person's life. You develop a lot of your personality quirks in the teenage years, and you might cause her a lot of grief about her sexuality that could cause her to be very depressed and embarrassed, as well as hurt her ability to trust partners in the future. If you were outed to the general population somehow, then even if she is okay with you being trans she could face a lot of bullying and harassment from fellow students. Other kids might declare her a lesbian, and she might not be able to convince them otherwise.
I know that these things might seem a little "transphobic," but since relationship is a two way street we have to accept the realities of our society rather than talking about the perfect world we WISH it could be. And, in reality, a person should be able to decide whether or not they want to be a part of this hot bed of political craziness, ESPECIALLY as a teenager when emotions are so strong and decision making processes are still developing. A really bad incident could scar someone for life.
If you were an adult talking about dating an adult, my opinion would probably be more liberal. A thirty year old has a lot more social development and general life experience than a teenager, and would likely be able to handle a situation where they discovered their partner was trans (before any sexual situations occurred) better than a teen. They also are not trapped in the fish bowl that is high school society and would likely be able to avoid any future interactions if they wished. Also, their friends would likely not be your friends (you would each have your own set of friends), so their other relationships would not be risked. They might question their sexuality a little, but usually by a certain age you come to understand that better and are less likely to have a crisis over it than a teenager would. Overall, it is a different situation.
My advice to you would be to become FRIENDS with someone you are interested in, feel out how they feel about dating trans people, and if they are open to the idea then consider coming out to them. This seems like the safest way to date in high school without risking pain on either side of the equation.