You also gotta factor in that alot of japanese males have phimosis or aren't circumcised. Another thing most of the males if they're in college or the work force I believe they only bathe out of their own volition and not a daily thing like those of us in the West or other type of countries that bathe regularly.
I agree with Smuggins. The Japanese bathe or shower at least once a day, so they're actually very clean. The prevalence of "stinky dick" in hentai is a reflection of the Japanese obsession with every aspect of sex---all the smells, tastes, and sounds (especially the sounds, hence the profusion of sound effects in hentai). Body odors are exaggerated for erotic effect in the same way that dick size and cum volume are exaggerated.
ahem, (puts on glasses), I think this is an interesting point. But I think that it depends alot on several factors. Bathing, personal chemistry, cut or uncut, and finally social stereotypes.
Personally, I think the last one is the real culprit, with the idea of an exaggerated smell giving the signal that he is virile. Similar to how the amount of ejaculate is exaggerated, the idea is of detail is to emphasize cultural ideas about virility and masculinity.
...Erm, I'm pretty sure cocksleeves aren't combustible trash?
Still, the good old 'caught masturbating' type stories followed by...blackmail, sort of. Except he offered to leave after making her cum once, although that was probably just to make her beg for it.
And that ending...The way he says it makes it sound like he's doing it for her, but I'm sure in reality he, too, wants to 'lend' her his dick more.