Because apparently fangirls prefer their male characters in estrus.
I got a chuckle out of this Google search, because the concept of Sesshomaru going into heat just doesn't work.
Unless he's secretly female or FTM trans.
For those who haven't looked it up (say, the people responsible for that search result), surprise! It doesn't simply mean you're horny.
It means you're physically female, and your body has declared that it's ready to get pregnant, in a way that male biology can't imitate.
So, sorry, fangirls - it looks like you can't count on the female reproductive cycle to get your Mary Sue into Sesshomaru's pants. You'll have to actually give her a personality and - gasp! - maybe even write some character development and a PLOT in order to accomplish that. Good luck!
Have you noticed any other fan-bloopers like that?
If you've got a bizarre and amusing "Fans Fail Biology Forever" story to share, I'd love to see it! Just post it in the comments so we can all enjoy it.