I am straight, but I knowingly kissed two trans-women
Does your gf know about this?
I didn't tell her. She wouldn't have minded, but I rather have some secrets for a while
Even if your girlfriend doesn't mind, you should be kissing anyone if you don't have feelings for them.  It could be construed as cheating on your gf.
It's all right. I don't feel gay and no one needs to know
Even so, regardless of how you feel about it, being physically intimate with people when you have no desire for them is dishonest.  These women that you were cavorting with were probably just having fun, but I still advise against this kind of philandering, especially when you are in a relationship yourself.