Thanks for suggesting Unexpected!
There are actually a couple of times during the story that Spike suspects Buffy of having an affair with Angel - one near the beginning when she gets pregnant and one much later (Actually in one of the "Life Goes On" stories).
As you said, even when he believes that the baby is *supposed* to be his, since it was done with magic involved, he's not 100% sure that it is ...
Thanks so much for the recommendation!!