I don't think it's immature or somehow non-adult to be frustrated that your own choices and decisions are relegated to a secondary status by the presumed elevation of other choices.
As one non-parent to another, I respectfully disagree about the immature part.
1. You're right that no particular individual needs to become a parent in order for the species to continue. I for one am not suggesting that you for one become a parent. Your choice not to is perfectly wonderful and there's nothing wrong with it. However, someone has to be a parent, otherwise the species will not continue. If there are no parents, eventually the living will die and the species will disappear, correct?
2. I have observed in others that being a parent does cause a person to mature in various ways. These usually include a shift towards a more giving perspective, and the development of some ability to recognize another person's viewpoint. I think in part that the experience of caring for initially helpless children causes these forms of growth in most parents. Your statements about your friends do not include a giving perspective, but instead a static me-vs-them-they-should-give-as-much-as-me perspective. Similarly, the emotions you express seem to be based on seeing your viewpoint but not theirs. In these 2 technical respects, which are meanings of the term mature, I submit that your expressed views are in fact not mature.
3. The term mature also implies that one's view changed from one state to another, in a process normally described as maturing, in which one learns from experience to take the "more mature" viewpoints. Your specific complaint, which is very accurate, is that your friends changed and you did not. Technically, they matured. By immature, I don't mean to denigrate you, only to express that you did not change.
4. My own experience in life included caring for ailing parents (my dad got Alzheimer's, I became his guardian). I observed in myself that the initially difficult (boring, exasperating) experience of repeatedly assisting him perform or adjust to small daily tasks, while I took responsibility for many major matters, brought a surprising bonding between us that was not present before. I grew to love him more by caring for him. I notice a similarity between that and what parents seem to experience. My new viewpoint is different; it has matured. In that the newer viewpoint brings with it abilities that the old one lacked, arguably the new viewpoint is better. In that sense, I do view your current position as immature. However, I also think that you can mature if you choose, and could sustain enduring friendships by doing so instead of "losing" friends.
As always, you get to make your own choices. Best of luck. It is surely disappointing to make so many plans that do not bear fruit.