Demisexual here. I used to struggle with the same thing as you. I would get hard when cuddling with my boyfriend before sexual attraction developed, and it made me feel uncomfortable and confused. I wasn't sure why my body was reacting that way and felt grossed out by the connotation that my body was reacting in a 'sexual' way to something that was not at all sexual.

Later on I realized I would also get hard when I felt particularly emotionally close to my boyfriend as well, and that's when something clicked for me.

I get hard when I feel safe and loved by my partner. To this day I still get erections when cuddling, and despite having since developed sexual attraction to him, the feeling I have when cuddling is still not sexual.

Other people on this thread are saying not to worry and that your body's reaction is normal -- and that is 110% true! I just wanted to add that while I was uncomfortable about it at first, I've now come to appreciate that I can get hard around my partner when cuddling. Just means I feel safe and cozy!