On sexy girls! If I could visit a male body, I would not refuse sex with some girls ... Besides, it would be very interesting. Experience everything from the male side, emotions, sensations ... After all, no matter how much you tell, how not describe not all sensations can be expressed in words.
On those to whom her female body reacted before the miraculous movement. This also includes her various fantasies, situations, fetishes, etc.
The point is not in the field, but in the brain.
If this is a heterosexual girl, then maybe a guy. But, as everyone is well aware, there are other options for the development of events.
Even if we exclude all sorts of LGBT options, then a member of a woman in a man's body will have something to erect from. As, in principle, a man's penis in her place.
Even the very image of sexual intimacy, for example, in an abstract painting, can be arousing.
In general, the penis has the ability to erect on anything specific. Without your knowledge, in the morning, for example.