Myth.
Being sincere applies to all people whether trans, cisgender, etc. I feel you will enjoy a much deeper and more meaningful relationship if you are sincere and open about your feelings and whatever thoughts you want to share.
Based on your subject, it seems as if you are having inner turmoil about committing to a relationship. Maybe you are not ready to settle down yet? I don't know how you are being tempted, so the only thing I can say to you is do not put yourself in that situation if you truly have feelings for your partner. Your common sense, love and respect for him will win over temptations. As for your friends being so frivolous with their relationships -- just remember what your parents told you growing up "If your friend jumps off a cliff are you going to do the same?"...
you know the rest.
The few transgender individuals I know are in committed relationships except for one and she has really a strict code of morals she lives by and I admire that. It sounds to me like you got some wild girlfriends which can be fun, but -- BIG BUT (refer to aforementioned parent's motto). Maybe it's time for a small change of scenery where your friends are concerned. I state this only because I have not experienced and cannot imagine my friend's, or myself, putting their relationships in jeopardy like that.
I have been in two long term relationships and the second one includes my present marriage. I've always been beyond honest in my relationships. I cannot think of another way to be. This is definitely a topic for a girls night at home with lots of wine and giggling! I hope it all works out for you once your gather everyone's input.
