Interestingly, and to the great frustration of elven cross-species lovers everywhere, the rate of unintended pregnancy in respect to cross-species lovers is much higher than the average unintended pregnancy rate, especially when the female is an elf. This occurs even when protection spells (Most commonly, a simple abjuration cantrip that is a slight modification of Resistance and Prestidigitation), are used. Research indicates that both this protection spell and natural biological factors are to blame.
This protection spell boasts a 97% success rate at preventing unintended pregnancies. Of the remaining 3%, most of the cases result from a misuse of the spell (The spell, though simple, requires both verbal and somatic components, and a range of touch. Furthermore, once applied, the spell causes a slight prickling sensation on the area touched; although it quickly fades, it is certainly uncomfortable.) However, the failure rate more than doubles when the participants are of different species, and more than quintuples when the female participant is an elf (This study assumes that there are two participants of opposite genders.)
Oddly enough, the spell varies slightly for each species. There is a protection spell designed for humans, a protection spell designed for orcs, and so forth. Therein lies the problem.
It is common knowledge that elves are highly androgynous. It is the rare elf that exhibits obvious gender traits, and many adventuring parties which include elves never know the gender of the elf or elves in the party. However, this androgyny extends beyond gender typing into actual biological characteristics.
All elves have a slight stature and build. Their bones are much lighter than that of other humanoids, and much more flexible as a result. However, this makes their skeletal structure much less able to handle the abuse that other humanoids can weather. As a result, elves have a lithe body structure that appears effeminate even on males. This extends beyond the skeletal structure, as elves have different hormone levels than other humanoids. Male elves in particular have much lower testosterone levels than other humanoids. This keeps their body slight and light enough for their bones to easily handle, but also prevents a deepening voice, severely hinders facial hair growth, and sharply decreases sperm count.
This last one is the solution to the problem that plagues so many cross-species lovers. When an elf uses a protection spell on a male of any other species, a protection spell that is designed for elves, it is simply not powerful enough. A different protection spell, one designed for the males' species, must be used.
It is unfortunate that so few people know this. This new knowledge must be widely spread to as to prevent any more of these issues amongst cross-species couples.