Your mother's nudity is detected by your nervous system, which will influence all your organs, including your penis.

Your mother’s mature body possesses feminine curves, along with the marks left by time and nature—a combination that is hard to resist. Why is that?
Our friend Olfa Abdou attributes this solely to sexual desire, overlooking elements like sensuality, the sense of belonging, and the need for protection.
A son posing nude alongside his naked mother—where his erection is clearly visible—certainly points to an intense sexual desire for his own mother, but it also reflects a profound sensuality rooted in a sense of protection; he is in good hands.
The son’s erection also reveals his pride: he has a mother who is physically healthy and capable of being alluring—a courageous woman, ready for anything, unburdened by shame or complexes, stripped of all clothing save for her pubic hair. The message the son wishes to convey is this:
"My mother is by my side; I follow her instructions and advice. She is a truly mature, experienced woman—no mere girl—and she can protect me; I know I can count on her for a very long time to come."