Even if I cared about my partner's gender (I don't, I'm pan), if I found out a partner I assumed to be cis was really trans, I wouldn't care. I may or may not still feel sexual attraction, depending on the person and my mood. The fact that they "tricked me" by not wanting to out themselves to someone they don't fully trust (yes, it's true, most partners don't 100% trust their own) is just basic reasoning and logic.