We know that Love Potions are legal, simply because the twins sell them. Their public store is visited by 100's if not thousands of people a day, leading us to assume that if they were illegal,
A. they wouldn't be on the front shelves.
B. someone would have reported them by now.
Other notable instances are in book 2, Lockhart suggests that Snape brew up some love potions for valentines day.
We also know that, at least from year 6 on, love potions are banned from Hogwarts, since all Weasley Wizard Wheezes items are banned.
*this may only have applied to Weasly Wizard Wheeze love potions, others may still have been allowed.
Rita Skeeter while being a backstabbing(fill in expletive) typically uses truth to enforce her lies or claims.
Love Potions are of course banned at Hogwarts, and no doubt Albus Dumbledore will want to investigate these claims. In the meantime, Harry Potter's well-wishers must hope that, next time, he bestows his heart upon a worthier candidate.
(Goblet of Fire, Chapter 27, Padfoot Returns)
So in my opinion at least we can take love potions being banned as fact. Shes appears to be using the "Fact" that love potions are banned, to re-enforce her denunciation of Hermione which is the lie of her article. If it was general knowledge that they were in fact not banned this would undermine her whole report, and since her readers are all Hogwarts students they would catch that lie immediately.
In year 6 they learn about love potions in potions class, however, they learn alot of things in their classes they may or may not be allowed to actually do/use in school.
* Bonus, while I don't believe we know for sure if Romilda Vane was punished, by the rules at the time she did have a banned substance on campus, which should result in at least a detention. However, since it was Slugghorn who was with Ron/Harry, and they discovered that poisoned bottle, there is a chance that the entire thing was kept hushed up.