I want to chime in here from a lady's perspective! I've had it bad for my current boyfriend since I was 16 (now 25) and he was not having any of it when i was so young.
He's about 5'11", 140lbs, so a pretty slim dude, my friends, my mom, everyone used to make fun of me for being into such a bean pole.
We used to flirt and tickle and one day I brushed up against his erection and thought to myself that it was a decent size. I hadn't been with many guys at that point, but it really intrigued me.
Life went on, we grew apart but ended up coming back together a few years later when i was determined to fuck him.
He isn't one to brag about it, and since I was only the second girl he'd fucked he claims he hasn't heard the constant gasps of 'wow' but when I got his pants off that first time all I could do was smile.
To this day I still call it my 'happy surprise'.