So, we've had Summer Fae over this season... and we just danced the night away. I think you get the picture
Morning after effects tables, anyone? Perhaps some random factor to see how many of the covenfolk women will be adding to the population 9 months in...
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.
I'd would spend to much time on random factors and such but just go with the flow and what suits your stories.
... I think the players can take for granted that something will happen. No need to roll the dice. No, the point on which you can (and should) get creative, is whom to.
I agree that the people who will bring the best stories just got pregnant, but in case you'd like a benchmark, current pregnant chances are about 20% for a healthy woman and man.
Current birth rates (in the US) are around 13% per year, but are much higher in non-industrial countries.
Other factors in such a drunken party could be physical changes due to eating fairy food (perhaps a personality trait being represented by a physical change, e.g. a stubborn person with donkey ears).
The loss of ones purse, or something even more valuable, such as tailsman or familiar.
Unremembered marriage, or other promices.
Property damage, to either your property or by one of yours to others property.
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.