Most Games avoid a Chance of Conception mechanic.
(unless you are using The Book of Erotic Fantasy)
Ancient Magic on page 56 gives a clue ,
Unless magical abilities are being used ,
then the natural chance of conception with a faerie father would probably have a higher Ease Factor.
Any affinities that the father had with Fertility , Creo or Corpus may well lower this ,
or if the mother has a Faerie affinity due to a virtue.
The above wisdom is truth- if you're the SG, make the best decision for the story- tables and dice are for when you don't have time, or don't care, or it just doesn't matter. None of those seem the case here. It's your story- make it a good one.
The laws of Fae can be quite diff from what a mundane might expect- casual words spoken while under the influence of drink and celebration can be taken literally, as if an oath. Things of value (or seemingly none) might be traded/exchanged without permission, or simply dis/appear. Unopened bottles of drink might be "changed", to be experienced at a later time, or animals given unusual gifts.
And any person who makes a real ass of themselves could always wake up with the head of one.
Oh- and as any myth will tell you, when a fae, male or female, wants a pregnancy to happen, it just does.