I just checked, and it's really not avoidable. This picture is unavoidable:
There is no route to avoid it.
Yes, it's possible because I didn't see that image, so it's definitely avoidable.
Here's what I did to avoid anything sexual with her:
Earlier in the game, when you talk with the FMC friend who owns a motorcycle, you tell him your only interested in women.
Later, when you're on the couch in your apartment with the trans woman, she mentions that she has something more between her legs. At this point, you can choose to pursue her or not.
You can avoid any sexual content with her. I believe you can't completely dismiss her story-wise, but honestly, why would you want to? She's cute, friendly, and adorable.
