I'm a guy with 2 SDs (6 & 10) and a little girl who turns 2 on Tuesday. IT's working but there's so many points to make:
However, when we got together it was on the proviso that I would never have a child of my own. For most of the reasons posted elsewhere in this thread. The eventual decision after a couple of years (we were in our late 30s at the time) was based on a number of things, bot least that both girls had frequently and vocally expressed the wish for a new sibling. This included the older one poking her mum in the belly with a magic wand (a stick...) and trying to magic a baby in there, and the younger one getting sulky whenever we had to explain that mummy didn't have a baby in her belly.
Not that we would have let them be the deciders in that sense, but it was consistent enough to believe that their wishes were genuine.
The 'adult' part of the decision was based on her appraisal of how I'd integrated into the family in the couple of years I'd been there. I have an amazing bond with both girls.
The oldest one, only once during the pregnancy, asked if it would mean I wasn't as close to her and her sister. I drew her a diagram of the 'blood links' - so from her mum and dad to her etc. I then drew in how the new baby would mean we were linked now, too - she would bind us closer, rather than apart. She liked that.
We've tried to be mindful of how it affected them since. If one I'm with the toddler and mum is needed by one of them, I'll try and get the other involved in what I'm doing with the toddler for instance. Luckily they both adore her, even more so since she found her 'toddler speak' version of their names.
We worked hard on making sure I bonded with baby early on so she wasn't too clingy to mummy so that the girls didn't feel they'd lost too much of her attention. She's been a total daddy's girl since about seven months, meaning if the other two need mum's attention she could be passed to me with no fuss.
We will never do anything special (holidays etc.) without all five of us being there.
Our toddler is fantastic, but noisy and relentless, and I think the girls appreciate the peace and quiet at their dad's every other weekend.
Their mum and dad's amicable relationship post-split has been a very important factor. He's a deeply flawed man in many ways (hence the separation) but has been so good about the baby with the girls. Never said a word that would imply she is more loved, asks how she is. When we got back from hospital he delivered them home from his and was the first outside of the most immediate family to hold the baby.
My partner's made it clear that if there had been any risk of him using the baby to make things toxic for the girls, it couldn't have happened.
My parents treat the girls as grandkids (they already had one step GD from my brother) which helped.
The girl's paternal grandmother has babysat the little one a couple of times and is keen to have all three of them together when she's a bit older.
The short version: We weren't going to, because of many of the reasons listed. But we had a lot of positive signs and lucky factors that suggested we could, and we work each and every day to make sure it doesn't make the other kids feel excluded. I think the biggest factor in all of it is how awesome my two little SDs have been throughout though.