So what, I wonder, is the sinister attraction in making our favorite characters breed? Going, in fact, to ridiculous lengths to get them to do so? If the story involves one of the vampire characters, you literally have to invoke a miracle, either a canonical one like the Shanshu prophecy, or the intervention of the Powers That Be, or a purely invented spell or a prophecy or a something to explain why the cold dead seed suddenly ain't. Why go to all that trouble? (I'm not even going to go into mpreg. Not. Going. There.) And it the attraction is so powerful, why is it also so reviled a genre?
It's not just a matter of "This would make an interesting story." Most of the time, babyfic isn't particularly interesting to someone who's not invested in the ship being written about. Most of it boils down to X and Y have a baby, with or without magic assistance, and they are VERY VERY HAPPY, the End. The baby is a symbolic device which affirms that these two characters are meant for one another. The more miraculous the conception and birth, the better. Very often the baby possesses special powers of some kind, a further significator of the chosen couple's worth.
If you're not a fan of the ship in question, there's nothing for you in one of this type of babyfic. The sole purpose of the fic is to proclaim the rightness of the pairing. I wonder perhaps if this is one reason why baby Connor was so unpopular; his birth affirmed the rightness of a pairing (Angel/Darla) that had comparatively few fans, and a secondary pairing (Angel/Cordelia) which ME was only lackadaiscally promoting, whether due to their own fear off ticking off the B/A shippers or due to the mysterious problems with CC. There were a number of stories written at this point which tried to undo the Angel/Darla and Angel/Cordelia ties by postulating that Connor was 'really' Buffy's, and that Darla was merely an incubator, or that Angel would call upon Buffy, not Cordelia, to raise him.
(There was a corollary to this among B/S shippers in the brief fad for stories where Spike was assumed to be Dawn's 'father' in that she was obviously not a clone of Buffy, so the monks must have used someone else's DNA in addition to Buffy's to make her. I think there were a few stories where Angel played the same role. I've always thought that this was an intriguing basis for speculation, but that the logical male for the monks to use would have been Riley, as he was A) alive, and B) Buffy's current boyfriend. But logic has little to do with this kind of thing; for the monks to use Riley would not have affirmed the rightness of the chosen couple. (Unless you're one of the six B/R shippers out there.)
Of course there are baby stories which are (and yes, I know this is a subjective term) good stories. The aforementioned Baby Blue, Dutchbuffy's Crossing Shadow River, Yahtzee's I-95 (Am I getting that title right?) Those are just the ones I can remember off the top of my head; there are others. But I would wager that a list of good baby stories would be shorter than the list of good examples of almost any other kind of story. And many of the good baby stories are not baby stories per se, they're stories which have babies in them. (Miss Murchison's Chiaroscuro series, Herself's Bittersweets series, etc.)
Part of the trouble with Jossverse baby stories in particular is that very few of the characters are particularly well-suited to be parents, or indeed, express any desire to be parents. About the only characters who could be said to actively want a family are Anya, who out and out tells Xander she'd like to have a child some day, and Angel. He mentions the importance of children several times to Buffy; he feels that his inability to give her any is a major stumbling block to their relationship. He is portrayed several times on AtS as getting along well with young children. When he finally does become a father in canon, he takes to it like a duck to water, and considers his relationship with his son the most important thing in his life.
Other characters have far more ambiguous attitudes. Xander is terrified of becoming a father. Giles is deeply ambivalent, at once loving Buffy like a daughter and trying to distance himself from truly playing a parental role with her, much less any of the other Scoobies. Buffy tells Angel that his infertility isn't an issue for her, and later, when forced to take on a parental role with Dawn, goes about it with a dutiful lack of enthusiasm, see-sawing from over-strictness to neglect. She does seem surprised and not unpleased to learn that a past Slayer had a child. The majority of the characters never address the issue at all.
The characters who do become parents in the course of the show, literally or symbolically, generally suffer for it, and/or make their children suffer. Joyce and Hank marry young, possibly because Joyce is pregnant with Buffy, although this is never made clear. They quickly grow apart, eventually ending in unhappy divorce. Joyce is the best parent on the show, and even she is portrayed as clueless about Buffy's life and unable to really accept her daughter's calling. Angel's son puts him through hell, and vice versa; eventually he realizes that Connor can only be happy and whole if he's not Angel's son at all. Giles betrays Buffy multiple times. Darla kills herself to give birth to Connor. Buffy hates the responsibility and drudgery of caring for Dawn. The only time parenthood is shown as rewarding or desirable is during Angel's brief relationship with baby Connor, and in Darla's deciding that Connor is worth her life.
The majority of babyfic seems to involve Buffy, which makes it doubly hard. It's up in the air as to how Willow or Anya or Tara might handle motherhood, but Buffy has already taken a stab at it, and the results weren't sterling. Buffy is a dutiful, responsible person. It's easy to imagine her taking on a baby if she's convinced that she has no option but to care for it. It's much harder to imagine her liking it, or being particularly good at it, given her general attitude towards taking care of Dawn. Yet there are scads of stories in which Buffy's attitude towards the unexpected arrival of a miracle baby is to be overjoyed, or at most worried for a bit over whether she can get the father to marry her and make an honest woman of her.
There are various ways to get around this problem in fic without simply ignoring it or bending the character out of all recognition. Set the story five, ten, fifteen years in the future, when Buffy's older and her biological clock is ticking. Make her partner the primary caretaker for the baby. Be really radical and have her give the baby up for adoption, or even abortion--though in practice, I've only seen this option used once, and it was in a Buffy-bashing story where the fact that she didn't want to be the barely-employed, single mother of a possibly demonic child made her a horrible person. (I'm the world's biggest Spike apologist, but a story where a soulless vampire who's cheerfully eaten thousands of tiny precious babies for hors d'oeurves lectures someone on the evils of abortion is, um, a trifle much.)
Or you can tackle it head on, and make Buffy a struggling, maybe even unhappy mother. This, of course, goes against all that a babyfic should be for many people who like babyfic. To make the parents less than blissfully happy is to cast doubt upon the correctness of them having had the child, which is to cast doubt on the validity of the pairing.
So what does all this add up to? Damned if I know. It does make me feel like I'm walking on eggshells when I get to the portion of my own personal story timeline which involves characters having kids. Have I made it believable that they could do so and be moderately successful at it? To what extent am I following the dictates of the story, and to what extent am I indulging some weird fannish obsession or thwarted maternal neurosis? From the inside, it's hard to say. Since the story's not finished yet, it remains to be seen if it all hangs together. And doesn't make people barf. *g*