Couldn't a boy simply fly into Girls' dorm?

I have trouble believing that if stairs could turn themselves into a slide, that there would not be some other form of charm or enchantment on the stairwell itself to prevent such an action.

I am unaware of anything even remotely canonical that would suggest this though - as a franchise, Harry Potter kept itself remarkably clean of any real lewd/suggestive material, and that Marauder's Map tongue-in-cheek joke aside, I can't even think of anything in the movies or books that was remotely suggestive of students having those types of relations.

He bounded forwards, pulled open the door and set off up the spiral staircase.

Harry Potter and the Order of the Phoenix - p.314 - Bloomsbury - Chapter 17, Educational Decree Number Twenty-four

I don't think a broom would fit up the staircase. Possibly through the windows, but I would imagine they'd be too small.

On the whole, though, I think providing a simple, sensible, almost fun precaution is one thing. I don't think taking an obvious mitigatory step implies that the founders' foresaw a need to barricade the girls in and somehow overlooked an obvious loophole. I close my windows and lock my front door when I leave the house. I don't have a moat a drawbridge and windows too narrow to get through!

Yes, they can. If it is emergency, boys, especially prefects or headboys can fly towards girls dormitory. The magic in staircase is just a warning for boys that they cannot enter there without emergency as girls want more privacy compared to boys and puberty controls boys more than girls. However, if a boy use broomstick only for fun or less trustworthy reasons, he will get punishment