I think novelty plays a huge role in what we find sexually attractive. Regular penis in vagina sex in just one position would get boring as **** after a very short while - thus you mix it up, with positions, foreplay, different things, toys etc. Anal is just another novelty. Humans seek out new experiences.
A lot of guys want to try anal just because it's something they don't usually do with a partner or haven't done before - it's something unordinary. Also supposed to be tighter (which is almost always appealing, unless you're particularly well endowed). There's also other things factoring into it like the taboo factor, forbidden, etc, and porn has definitely fetishised anal. In reality, humans have been having anal sex for thousands of years - documented by the Indian kama sutra, the romans, greeks, and ancient egyptians.