I’m a 33 year old only child, and also childless by choice, and I identify with a lot of what you’ve said. Probably the only bit that’s not the same is that I love animals – I’m a vet so it’s kind of a prerequisite of the job, and I have 2 dogs of my own who I utterly adore, but they’re not ‘child substitutes’, they’re way more awesome than that 
I know my Mum would love me to have kids, but luckily she’s very cool about it and would never guilt trip or pressure me over it. She wants me to be happy above all, and she knows that, thus far anyway, settling down is just not for me. I travel a lot, usually 3-4 months every year, and while she and DSD are happy to have the dogs, I’m not sure that I could ditch a kid with them for a few months at a time while I go ice climbing in Patagonia or trekking to Everest Base Camp. Even if she did pressure me to have children though it wouldn’t be the right reason to do it, although I get that it would be difficult and I would feel a degree of guilt – and also undoubtedly resentment – about it. It is more difficult when you’re an only child, but it’s still not a reason to do anything so completely life-changing if you don’t really want it.
A lot of my friends have children now, and although I like most of them well enough, in small doses, and see how special they are to their parents, I’ve yet to feel remotely broody or maternal myself. It may yet happen at some point that I do, I’ve never said never, but so far I can’t imagine it. I’m in a relationship but he is about as ambivalent as I am about having children. I also have quite a few friends who are childless by choice, some are my age and some are older. Some of them may go on to have kids at some point, but a few are very certain they will never do it, even more so than I am, so I don’t feel like an odd one out in my peer group. I have ended a previous LTR due to him wanting marriage and children and me really not, and I am always very upfront with people that I may get involved with about it as I wouldn’t want to be in that situation again. Because of the kind of people I tend to attract and be attracted to – mostly travel obsessives like me - it’s not usually an issue though.
I’m extremely happy with my life and very confident about my choices and decisions, so other people’s opinions don’t really bother me and never have done. However you’d have to be blind or stupid not to notice a definite attitude from some parents towards people who choose not to have children. If there are any phrases that ever make me want to smack someone in the mouth it’s the old ‘you never understand real love until you have children’ ‘life is so shallow and meaningless until you have children’ etc. etc. It’s fair enough if the pinnacle of your existence is having children, but some people can’t seem to grasp that not everyone feels like the main/only achievement of their life would be having procreated. I don't feel the need to 'achieve' via my offspring, if I want to do something I'll do it myself. I think those types think that I’m kidding myself though, which is fine, they can think what they like, but if I was in a situation where I desperately wanted to have kids but couldn’t then that attitude really would be horrible to have to deal with. The whole ‘have children because you’ll need them in your dotage’ thing also strikes me as rather selfish. There’s also no guarantee that it’ll happen anyway, regardless of how well you bring them up – people emigrate, people get sick themselves and even die, people have their own families/other priorities (the selfish bastards!!), so when push comes to shove they may well not be there, or not want to be there, to look after you.
As for regret, well, that’s always a possibility, but that’s the chance you take with anything in life. I can only make decisions based on how I feel at the time, so ruining my present by having children I don’t really want because I’m afraid that I might regret not having done it at some point in the future would (to me) be ridiculous. Not to mention unfair to the children. I’ve seen so many previously interesting people seemingly lose their identity after having a child, turning into the classic ‘baby bore’ (not everyone, but it’s a definite trend), and the thought of that happening to me terrifies me. Although there is part of me that would really like to piss off the uber judgemental holier than thou types who like to spout off regularly on Facebook about breastfeeding/slings/co-sleeping – sometimes it would almost be worth having a kid so that (regardless of what way I actually chose to bring it up) I could horrify them with regular status updates about my poor un-attachment-parented bottle-fed child. Or post that it’s ok but I still prefer the dogs – although that one would probably be true 
Jeez, I just realised how long this is, sorry for the essay! On-call last night so not at my most concise today 