I would say it depends on the situation how the "boner" incident would make me feel. For example, if you have just ended a 2nd date with a young lady, you kiss, and you get a boner, you would probably not want her to know that...dont get me wrong its very natural but that could be interpreted (on her part) that you're overly sexual.
If this young lady is your girlfriend (or something like it...haha) its a beautiful thing and i dont think it would bother her in the least.....it doesnt bother me in the least, i quite enjoy it.