I see no particular reason why they should be illegal. So many people claim to understand the universe of wizards, but they fail to accept one thing.
No matter how many times they read the books, they were born muddles, they have lived as such, and bringing their social norms into an alien world of magical beings is completely useless.
So what that love potions are not illegal? In the Harry Potter series, we can find too many potentially harmful devices, behaviors, and standards not compatible with ours that pointing only one is ridiculous.
I wonder what makes those complaining pussies so sure about them being right? There are countries eating cows and countries deeming cows as sacred, yet nobody seems to complain about this stark difference in approaches, but love potions in worlds of wizards should be banned because they may lead to nasty consequences, similar to throwing a sleeping pill to someone's wine.
I would like to point out a few things for you. As muddles, we have no idea how wizards think. The books indicate that wizards don't care about us at all, so why should they care about muddles' opinions on abusing magic remains a mystery.
Moreover, J. K. Rowling used the scenes with love potions to entertain her readers, and thankfully, she disregarded the lot obsessed with the urge to dig up something deeper, which never existed in books for children.
And yes, her famous series is completely asexual, so we may conclude that wizards do procreate (similar to as I saw in one old British sketch) by drinking hot cocoa and keep themselves nice and warm, waiting for the house elves to deliver a tiny wizard baby for them.
The case may be the wizards would find our muddles' nasty insinuations disgusting and inappropriate. Murder and torture? Why not? Sexuality? Despicable! If I am not mistaken, the US film movie rating system has been based on similar principles.
Or perhaps, given the diminishing number of wizards, they would even encourage using love potions if it guarantees the birth of another small pinky wand-waving piglet, I mean an adorable little magician, sent promptly away to Hogwarts.
Anyway, there is a real-world waiting to be corrected, so why lose your time questioning the fictional one?