There are many reasons why you would want to keep your relationship with a TS girl a secret. Friends and family assuming you are gay, ostracism, criticism, and employment issues are also a risk should your relationship become public. While openness and honesty are important, there are sometimes good reasons to keep things under-wraps, at least in the short-term.
Obviously, if this is not your first relationship with someone who is transgendered and everyone already knows about your previous relationships, normal dating rules should apply, but if this is your first and the revelation would be a “coming out of the closet” (so to speak) you might want to tread lightly until you know for certain what’s what.
It’s important that this situation is not framed as you being ashamed of your mate – very important. Your T-Girl has likely already dealt with a lifetime’s worth of shame and rejection, so you adding to it will only serve to increase an already inordinate amount of accumulated hurt. If the relationship has progressed to the point where you know she’s “the one,” keeping the relationship a secret will have a corrosive effect on your relationship, and you should seriously consider normalizing the relationship by finally telling your family and friends and letting the chips fall where they may.
If the relationship is still in the early stages and you’re unsure if it has a future by all means feel free to keep it private, but realize you are sending a clear signal about your assessment of your relationship status to your girlfriend. Keeping the relationship on the “down-low” is just as easy as keeping any other relationship a secret, though your friends and family will certainly be curious as to whether you’re dating someone or not, it’s easy enough to tell them you’re taking a break from dating, at least in the near-term.
Depending on the size of the city in which you live, there may be trans clubs and restaurants you can go, if you live in a less populated area, avoiding certain places that you know your friends and family frequent would be advisable. Like any affair, there is always the risk of discovery, so best you and your partner discuss in advance what the plan is if and when your relationship is discovered so as to not incur unnecessary trauma during what may be an already traumatic experience.