Purpetrated by hetrosexual men or homosexual men? I have never done any reading on this particular slant BTW, so I it'll be best guesses!
It's like the question that people ask about rape; if it is about sex and desire, why do men rape old women and very young girls?
The only answer to that is to say that statistics tell us that women of childbearing age are massivly over represented in rape stats, the highest being teenagers and early 20s, then gradually getting smaller until a drop off of rare occurance after menopause.
The stats tell us an average from which to buikd up a picture of the 'norm' - it cannot account for sexual pathologies, such as paedophillia or the opposite, all it can tell us is that these instances are rarer, and out of the ordinary of observed victim demographics.
Obviously, people who prefer infertile looking women are less likely to be successful in passing on their genes to the next generation so it would be viewed as an anti-adaptive behaviour - but much thinsg are these days, such as our liking for fatty salty food which is killing millions of us every year.