"禁断の愛 : Kindan no Ai" sounds more appropriate in this usage.
I can't grammatically explain why 禁物 is not good, how ever it doesn't sound poetic. 禁断 sounds something more fatal.
There's no Japanese expression 禁愛, it's a word of coinege, or possibly a title of a novel.
It sounds for me, something like "no smoking", "no parking", and "no love", sort of tings.
It sound more like "Love affair and sexual activityies are prohibited in this area.", rather than "Forbidden Love".