A straight man boning up when seeing another stiffie is as normal as blueberry pie. I have an ex-Marine bud who also like to hang out naked. He bones up when he sees mine... and he's definitely not gay. I get hard because he's beautiful and I think he enjoys the reaction it gets from me.
At Playalinda Beach in Florida it happens all the time. In high school (a long time ago and long before you'd be branded as GAY (the word hadn't been invented yet), if a guy got hard in the gym shower, most all of the rest of us in the gang shower did also.
One day you might find yourself in a conversation like the one you were in when the other guy is admiring your boner and wants to touch it. It certainly doesn't make you gay if you let him and his touch feels good to you. Even if this progresses on toward mutual masturbation. It's just a "guy thing." Enjoy the moment.
Frankly an erect penis is erotic I think whether you are a straight, bi, or gay man. Partly I think it is due to its purpose for being erect, but knowing that the man with the erection is sexually aroused adds to the turn on. I dont see it as an issue if it made you aroused, chances are it would have made me aroused, and I would have enjoyed it for the moment!
A cynical person would think there is some sort of agenda here to discourage nudism by implying its a sexual experience, which it isn't. Its happening so often with naturism.
It would be understandable to thing that based the these overtly sexual questions and comments