The truth is somewhere in the middle. Being elastic and able to revert to a previous shape doesn't mean you can't be deformed. Skin is elastic, but obviously it doesn't immediately snap back if you lose weight, especially if done quickly like a liposuction. You can overstretch a rubber band and it can snap, or sometimes if you overstretch some bands they just don't return to their original length. They can lose resistance over long-term use. This is less so with biological structures due to self-repair capacity, but still happens.
Using a "think of your wife" analogy is misleading, because obviously if she's fucking the same cock it's going to fit rather snugly around it. I'm talking about a massive difference in girth though. Stretching to 5 can keep you feeling snug for 5, but it won't keep you feeling so snug as a 6 if she's doing 8s. Especially as they age and skin loses its elasticity, and if they aren't the type to practice kegals and maintain muscle tensing ability for gripping.
If a woman fucks HUGE cock, she'll probably rely passively on her non-muscle soft tissues (I want to say 'fascia' but I'm no anatomy expert) for tightness while focusing on relaxing her muscles to accomodate a large partner. By those being stretched by big ones, she'll lose passive tension, and lose the habit of generating active tension.
I'm not saying women can't bounce back, just that it's situational. Like you said with the baby, there was a recovery period. How much it changes depend on how much (and probably also how regularly) it gets deformed.
Babies, dildos and fisting are probably more of a threat to tightness than even the most girthiest meatsticks though, so thoughts related to that are probably less of a priority than most think them to be.
Virtue's kind of a subjective issue here, everyone has different ideas of what they consider virtues or vices. Whether or not fucking makes you fucked in the head is more about the quality of partner than the quantity. I do get that with large quantity we tend to assume people are choosing lower-quality mates though. It's probably true, but I worry we focus so much on shaming people over number that we ignore people who have low numbers but equally low (or even lower) quantity.
Surely a woman who bangs a thousand 6s isn't as bad as a woman who bangs a single 5. I'm not talking in terms of looks, btw, just whatever means by which we measure quality.
We could say something about the quality of relationship, but it's the same thing. Being monoamorous is no guarantee people have a good relationship with their partner. Casual partners may not be thoroughly involved or open but they could still coexist on good terms.