A question about nude beach ettiquette?

I am 34, male and have been going to the clothing optional beach the past couple weeks with my friend Ryan. He is ok with family nudity and the last time he brought his 14 year old daughter Kristin along. She didnt get nude but wore a bikini. Well the three of us were lying on the huge beach towel on our backs and u guessed it, I got an erection. I was embarrassed cause I couldnt hide my 3.5 inches and Kristin did look. Ryan didnt say anything but I am not sure he was happy with me about it. I went 5 other times with Ryan and it didnt happen. Should I be ashamed of myself?