I'm a little worried over the fact on how she doesn't have any underwear on.... This is how you classic Hentai's start.

No. Classic Hentai starts with a sex scene. THEN they touch on whatever plot (if any) after that scene. ^_~ This is just subtle fanservice. ^_^

why do artist draw girls bent to an impossible to reach degree and their back as well?

Because as impossible as it may be (when you actually bother to think about it anyway), it's hot/cute/sexy/appealing? Besides, maybe she is actually that nimble/flexible. THere's weirder stuff in real life.

Whole background perspective is weird here... or theres no perspective at all. And girl is just half-lying on a table. Maybe strange pose but common for anime style.