Chyos walkthrough says you can fuck melissa when EITHER you get 15 affection or 5 corruption.
But her affection is capped at 10.
also, the fact you need to "come back tomorrow" to get the hint for a map is rather frustrating.
I already dug up over 10 spots and it just really drags on to have to skip yet another day each time I want to hear her hint.
edit:
ok seems like her affection cap goes up from 10 after you find the 3rd map piece... but it only goes up to 11A 11C.
Maybe the 4th piece raises it even further?
so, need to update the walkthrough
this line should note how far you need to progress her quest before you can actually raise her affection to 15
assuming it is possible. I have not finished her quest yet. it is very slow with the maps.