I got married after three years of living together (no children). Almost everyone I know has lived together before marriage and some of have been together many years. I don't begrudge any of these people the trappings of a BWW and honestly, most of the people took a while to get married because they were trying to pay for it themselves and not go bankrupt in the process.
I had two bridal showers, one for each side of the family. I also had two baby showers, one for friends and one for family/my mother's friends. I didn't throw any of my showers, other people did, I just provided guest lists (and there were no overlaps). Most people I know, regardless of how long they've been together, have at least one shower, if not more.
You said that if they had gotten married before living together/having children you would have had no issues with wedding/shower gifts, so I do think you're being petty begrudging it now. Maybe a registry isn't to your taste, but some people like it. I know I used my registry as a chance to finally stop using hand-me down appliances and get some nicer stuff (I also have relatives who are opposed to cash as gifts and inquired several times about registries).
I agree with one set of showers per person. I don't like showers for second babies or second wedding (though there are exceptions to the rule), but I don't begrudge a first time bride a shower, no matter how long the couple has been together or how many children they have.