I was looking up pictures of fit pregnant women and it's not weight gain. The stomach gets that way because you've got 8-10 pounds of baby and an expanded uterus pushing out against your skin and abdominal wall. It's a lot of pressure on your tummy and your skin. Some women say it feels like their skin is ripping apart. Your abdominal muscles literally divides (called diastis recti) and leaves you with that pooch look that people mistake for fat. If you look at pics of tiny women at 36-40 weeks it's easy to see how stretch marks develop. Sounds like it's not really preventable unless you undereat and have a retarded baby so you don't look bad later, but what selfish person is going to do that? I saw pics of a woman who trains for marathons and was probably 96 lbs prepregnancy, maintained a low reasonable weight throughout, was back to running 12 mi a day a few months after giving birth and she still had stretch marks and loose wrinkly skin.
I think women are okay with flaws on men just fine. That whole dad body trend is still kicking and bald men still have pretty and successful wives.
First, i think i recall that dadbod stuff being started by some college chick(or dude).
Second, its pretty much genetics first, maintenence second, but no one says stretch marks cant happen. However some of the worst examples were simply folks who didnt take precautionary measures during pregnancy.

however, the health of mother and baby are far more important, so im not saying this as some call-to-arms for women to spite themselves just to prevent stretch marks. In any case, my issue is with calling someone a "real man" over something as trivial as dikking you down in spite of stretchmarks. Men are out here smashing chicks with all manner of ailments just to get that nut. You cant say on one hand "men will fukk anything" and then on another be all "yaaaaaaaaas" when a guy who isnt interested in you makes a social media post about loving the stretch marks. Interesting thing to note, he also chose a picture of a chick who is clearly fitter than average, thus making the stretchmarks far less of an issue anyway
Course people aint gonna trip out if Ciara has stretch marks, but Kiara who works at Food-for-less and has long since lost her shape

Good luck.
And we all know women choose less than stellar men often. Its about the feelings for yall.