This is a strictly sociological question but far out theories will be laffed at!
I don't know why, I just know I do. Big ones, small ones I like them all.
I suspect the attraction to them is rooted in evolutional biology. In many primates the beasts swell during estrous signalling to the male that the female is ready to breed. Even though human females have evolved the ability to disguise estrous male humans still respond to the signal.
Well... the question doesn't stipulate they are attached to a woman... could be moobs!!!
@HeyHiHullo , it could be but, that answer still works for that, too. Sexuality has little to do with biology; years ago I did an experiment with both straight and gay men as the subjects, where a woman in a room dropped her top and mens reactions were recorded. The reaction of starring time was virtually identical as were their expressions. There was some flaws in the experiment but, overall it did suggest that the reaction was not based on sexuality.