There was a programme on Channel 5 about it not long ago. It was really funny. Now, here's the point. The argument against sexual behaviour which is anything other than heterosexual from narrow-minded bigots is that "it isn't natural", and that will normally be dismissed and narrow-minded and bigoted.
But in the case of "mechaphilia", as it is called, the object of their affections isn't natural, so how can their desire be? They still have it though. They're still that way inclined sexually. It's still very real.
I guess it comes down to what is meant by "natural". To me, just intuitively, the argument that being attracted to cars can't be natural because cars aren't natural is a pretty good one. Looking at it another way, people will often claim "I didn't choose it - it's the way I was born" to justify their sexual behaviour, but surely a person can't be born with a sexual attraction to a car, simply for the reason that a car is a human creation.
That's a pretty deep philosophical debate, I guess - did cars exist on a metaphysical level before we invented them? You'd need to answer yes to justify the "way I was born" claim, I think. But then the contrary argument, for zoophilia, for example, doesn't seem to run as soundly, i.e. anything along the lines of "sexual desires for animals are natural because animals are natural".
I'm digressing, I think. The key thing is, while it appears evident, for the aforementioned reason, that mechaphilia isn't natural, the actual reason for it not *being* natural remains undefined. To clarify, I could say I know an apple is green because it's not red or yellow - that's a reason for me *knowing* it's green but it doesn't give a reason *why* it's green.
So you say, okay, there's something undefined which makes mechaphilia unnatural. Could this undefined thing therefore be applied to other sexual preferences, and if so, which ones? Zoophilia? Paedophilia? Homosexuality?
Note, this isn't a "Which sexual preferences are more likely to harm other people/animals" argument, nor am I starting at a point where I think any of the aforementioned (other than mechaphilia) are "unnatural" and trying to justify that. I don't at all (definitely not homosexuality, anyway. Am I safe? I think I'm safe). I'm just taking the premise that some sexualities aren't natural because they involve attraction to unnatural things and wondering how far you can go with it.