Okay, how exactly is this anti-trans? The joke is simply that the man already went and said "I'm okay with that" before realizing she was talking about transsexuality and not religion. Most of the humor just comes from the clever word-play on "Christian".

What anti-trans message does this joke really carry? Does it imply that the man isn't fine with her being trans? Maybe. Does that make the joke anti-trans? Not really. That's mostly just the reader's own assumption. The joke doesn't explicitly mock trans people in any way.

This is a joke featuring a transsexual person, that draws most of its humor from the fact they're trans in a non-mocking way. That's quite different from, for example, most jokes about jews or black people. It's situational comedy, not offensive humor.