Define 'natural'?
If, despite their full knowledge of the situation, they stil fall in love why is that not 'natural'? It's not 'artificial'. There are no external forces influencing it.
By your definition is homosexuality 'not natural'?
Let me think this out for a minute.
Is incest natural? It's appeared in practically all human societies, since the dawn of time. It's been an unspeakable crime in just about every society and civilization, too. It's practically the only universal law, the taboo against incest.
Nearly universal, that is. There have been
slight exceptions to incest- it's been accepted for religious, cultural, social or political reasons but only in the upper/ ruling classes, not for the population. Royalty in Ancient Egypt, royalty in Inca culture, the royal family of Hawaii. Ancient Greek myths are rife with descriptions of incest among gods, goddesses and other supernatural beings. Marriages to nieces and nephews were also accepted in ancient Greece.
It appears in nature, too. Horses are bred with their own offspring and grandchildren, etc. Maybe it's "natural" that the first people adults, teens and children turn to for sex are those in their own home or close community. The Eskimos traditionally practise some incest- makes sense in a community and lifestyle like that.
That it appears in nature and in human history does not make it moral or normal, though. Rape, infanticide, patricide, murder, cannibalism and bestiality have been part of humankind since the dawn of time. That doesn't mean we should accept them or tolerate them in any way. Rape is very natural and actually very understandable but it is still an unforgivable crime, whether it's men against women, men against men, women against women or women against men.
Is incest abnormal? People who do it may have something very, very wrong with them. Or maybe they are otherwise very normal. Maybe they are intelligent, sane and educated people who just happen to be having sex with their biblings, parents or children.
It's still beyond stomach-churning, though.