I didn't review all 70 previous responses, so if someone has already made this point, I apologize. in my view the situation was that you saw a guy get erect as he approached you and your female companion. my default assumption is that he was erect at seeing your companion, and that you were glad that he was admiring of your companion in that way, and also excited that he was openly excited by her as you (may have) contemplated her reaction to him, and the thought of them being excited by each other right in front of you. there are many possible triggers, since you don't mention her reaction, we can only speculate.
By my count thats five assumptions and one speculation, anyone could do that and come to a completely different conclusion. Whos to know whats really going on.
My wife...now ex...and I were nude on a secluded beach when approached by another male and asked us about being nude on the beach...we informed him that it was not a "legal" nude beach but many enjoyed this section naked. We invited him to strip down and join us...which he did. We were very open about getting our all over tan... as my wife sat with her legs open. Being shaved she was very much on display and enjoying the display she was posting. He became erect as he was openly staring at her gaping pussy....well...lol...so was I! I also became erect...he apologized and was going to cool off in the water when she stopped him and said it was quite alright to be aroused and that it made her feel good that she would have such a great effect on others. He remained seated as we were all quite aroused....it was very natural to enjoy watching each other as we moved to the next step on this deserted beach of self pleasure! It just seamed right to enjoy ourselves in front of one another!