@Nick
Obviously I should've been clearer, but since this is something I assumed everyone with a high school level of biology knows I didn't think I'd have to clarify (that and you're being a douche, so no cordial apology for you!) :P
ALL people have a bulbospongiosus muscle regardless of sex. This muscle helps control the flow of urine from the bladder. It also plays a role in erection, ejaculation and orgasm. Because of this it is harder for men to urinate while fully erect. HOWEVER, since the muscle contributes to the flow of blood to the deep dorsal veins and spongy material of the penis AND urine flow it is obvious that you cannot have an erection and urinate at the same time UNLESS the muscle relaxes, in which case you will urinate immediately and shortly thereafter the erect penis will disgorge the stored blood from within the penis. It may APPEAR to be erect while you're urinating, but it's only in the process of voiding blood from the penis. And unlike exactly after intercourse it won't expel the blood quickly as that would cause the urine to fucking blast out of your cock, and your dick is smart enough to know it's not a vagina which primarily uses gravity to expel urine instead of a more heady series of contractions.
Now, I did make a mistake. Erections aren't the main thing that keep boys from having nocturnal emissions, however it's been proven that they help growing lads from wetting the bed. However, it's also somewhat of a crutch as most girls stop wetting the bed at age six, while for boys it's seven.
Lastly, I can confirm women don't have penises as I have done extensive research in the field. This is a widely accepted theory that I thought required no explanation. Anyway, I had a lab coat and clipboard at the time of study, and I'd love to compare notes of my research to you, however it seems many junior scientists are engaging in similar activity in this very thread. So for the sake of science I suggest we get back to the topic at hand: wildly exaggerated claims to sexual prowess interspersed by one or two people who bother to tell the truth, that being that sex is pretty overrated and awkward.
But to answer Venetia's question, yes little boys get little chubbys.