I always thought that the 13 children was a bit excessive ..... like saying "Sam was 300% heterosexual despite his devotion to Mr Frodo.... "
As for the intermarrying of elves and men ... I might say that all women marry beneath them... lol but you could speculate that compared to the androgynous elven men the mortals might have seemed really hunky and oozing testosterone.... also Melian might have started a genetic tendency to marry down?
Another factor is the enclosed communities that they lived in (Doriath, Gondolin...) like changing schools at sixteen or going to uni and suddenly there are loads of guys around who you don't remember as ten year olds.....much more appealing.... probably a tendency nature encourages to widen the gene pool....