"Big Macintosh's genitals." Said Twilight, blushing at the last part.
"That sounds wonderful Twilight, please cast it right away, if that's no problem for you." Said Fluttershy, while Big Macintosh only offered a quiet: "Eeyup." Twilight nodded and her horn started to glow. She blushed even harder, as her magical aura started to engulf his balls. As embarassing as it was for him, Big Macintosh felt his erection growing, as the tingling aura touched his private spot. A bright flash later, the spell was done. "Alright you two, this should do it. Now, I can imagine you two want to get home now."
Both ponies blushed at Twilights words, although Big Macintosh still felt strange. He looked down under his belly and saw that his balls were bigger but also looked and felt strangely empty. "Uuh, Ms. Twilight, are you sure you cast that spell correctly? It does not feel very full."
Twilight was slightly confused at this. Shouldn't the spell have increased his fertility tenfold? "That is strange. Do you mind if I... test it?" Big Macintosh felt really uncomfortable now, but it was probably better to be safe, so he nodded and let Twilight feel his testicles to see, if there really was anything wrong with it. Indeed, when she felt him up, his sperm count seemed to be very low, almost empty. "That is strange, I will take another look at the spell and see what could have gone wrong."
Fluttershy and Big Macintosh were a little nervous as they watched Twilight read the pages again, even more so as their friend suddenly became very pale. "I'm sorry... I am so sorry." She almost sobbed, as she showed them a part of the spell, that she overlooked.
Warning
To create more sperm, the male subject will need to ingest living matter with his penis to convert it to sperm.
"Wait, what does it mean ingest?" Big Macintosh looked down again. Due to Twilights touches earlier, his cock was still poking out. This time, he took a closer look and saw that his urethra seemed to be opening and closing in a breath like rhythm.
"It means you have to eat somepony with your... penis. It says the effect cannot be reversed and that you can only control it, once you have done that at least once." Said Twilight, deeply distressed at the horrible mistake she made. "If Fluttershy gets to close to you sexually, you might accidentally ingest her and turn her into sperm." The three ponies were horrified at the idea. This spell was supposed to solve their problem, but now it seemed to be worse than ever.
Twilight still wanted to fix her mistake somehow, so she...