Do they have to be all happy through the entire fic? Because I think that might be a little hard. Because in most fics, especially all-human, they have some obstacles that they have to overcome.
But still I can recommend some that have also happy moments and are not too angsty.
Mercy: The Surrogate http://spikeluver.com/SpuffyRealm/viewstory.php?sid=1455&warning=NC-17What do you do when you find out that your wife has been cheating on you for the past year, with your so-called best friend and flown the coop back to London....leaving you with a hell of a mess! When you've just found out that you're about to become a dad, with you're wife's sister....who just happens to be your surrogate and the baby's biological mother...and you're the biological father. Mix that all up and what do you have...The Surrogate. Won Best Baby Fic at Lost in Spike Awards Round 6 It is a WIP but the ending is not too bad. That shouldn't stop you from reading.
Squawks: The pen is mightierIt is not online anymore as far as I know but I have a copy of it. Just contact me.
Buffy and Spike work together. Spike is more of the William-type and after a while they hook up and Buffy gets pregnant.Facingthesun: Having my babyhttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=16390One day in the life of William and Buffy. Flashbacks are included to show you how their relationship has transformed from a deep friendship to a loving and committed union where Buffy is expecting her first child. It is slightly angsty but still a very good fic. Probably even more so because of it.
A fic that is not so all-happy but still one of the best is "20 month". You should definitely check it out if you haven't already.
Machine: 20 monthIt's also not online anymore as far as I know. Again I have a copy I could give you.
Buffy and Spike share a drunken night then decide to marry after Buffy finds out she is pregnant. They don't love each other. They barely know each other. But they marry under the agreement to divorce one year and one month after Buffy gives birth in order to keep Buffy and the child away from Buffys abusive parents.