Of course since Lucilla is a product of my mind, I know what I was visualizing or imagining when I had her do this. On the other hand, being of the heterosexual male persuasion myself, I can't pretend that I can accurately imagine how the mind of a girl might work in this situation.
Now looking at Lucilla's life thus far, she is a slave, her life has been taken away, and very unfairly. She is being punished solely as an example, not as a result of her own actions. At this point she has been through five crucifixions, hanging naked on various crosses - not always the "X" cross we see here - and suffering until she was senseless and so weak she was near to death. So she has spent as much as 15 days during the past six months or so on a cross.
As a result, she's angry and defiant at the unfairness of her punishment, and after all of those days hanging naked and struggling in front of a crowd, nudity doesn't matter much to her any longer. When they were about to strip her for her crucifixion, she tore off her own loincloth and threw it in Antius's face. And now they are about to crucify her in the most humiliating way possible, with her legs spread on an "X" cross.
Lucilla has been hanging by her wrists since her cross was raised, during which I've shown several scenes, but those were instants I time that were of short duration. Here's a recap:
They raised Lucilla's cross and let it drop into its hole.
The hole was substantially larger than the base of the cross, so the cross continued moving forward until it was leaning, leaving Lucilla hanging by her wrists.
They let her hang that way for a few minutes before they began to straighten her cross up and fill in the hole. The shock of the sudden drop that jarred her shoulders and arms, plus the length of time she has been forced to hang, are adding up by this time. She hurts and she wants some relief, soon.
Now I think that a victim's natural reaction on any kind of cross would have been to try to search for some support for her feet. Most of the time, she wouldn't find anything substantial. But in this case, there are the lower crossarms of the cross. So while they are filling in the hole around the base of her cross, Lucilla gets one foot around and gets her toes onto the top of a crossarm. It's a bit of a stretch.
When it comes to getting that other foot onto a crossarm, it might give her some pause, but not because of the humiliation of so completely exposing herself, because she's about to be exposed like that anyway, and she's been naked in front of a crowd for days. She's had plenty of time to lose any sense of modesty she might have formerly had.
She does have some fleeting concern about her vulnerability once she puts herself in that position. The whole purpose of her crucifixion is punishment; with the temptation there, what might her executioners do to her? She makes the decision to take that risk.
Which leads to brief pleasure, although the humiliation of having this done to her before all of the onlookers is something she balks at, begging him to stop before losing herself in it.
And then, when she is almost at the point of climax, the pleasure suddenly turns into agony as Antius pinches her sensitive clitoris hard and twists it.
So yes, she had a choice, the humiliation of being in such an obscene position wasn't much of a negative factor, but the vulnerability it presented was. She made the decision to take that risk, driven by the prospect of some relief from the pain. She has made many such choices during the long days that she has already hung on a cross; whether to avail herself of the support of a sedile, trading the pain of the cross for relief at the expense of sexual torture.