Doesn't porn mean having two or more people indulged in intimate physical activities? Or even if it's one person, there should at least be some tool/toy or hands involved that are being used to achieve pleasure. Mere nudity, in case of animals, shouldn't be considered porn since they don't wear any clothes anyway.

Now, even if that cycle has something fixed in its seat, we can't know and hence can't assume that there is, because it has no visibility for us.