So I am going to comment on the OP before I read any other post. And of course because of the character limit, this will be in two parts.
For starters, we're encouraging (from elementary school!) children to identify as other-than-hetero. The only explanation for the massive swell in non-binary/trans/etc youth (beyond all historical levels) is we've made it somehow *cool* to be "other than het".
I don't see where Heinlein would have seen this as a problem. When you look at his stories about group families, particularly with Lazarus and Michael, there is, at the least, no discouragement from same sex interactions. Furthermore, he does place sex and reproduction as separate things. While all of his stories do focus on male/female relationships, I think some of it was him being a product of his time, and some of it being not pushing the envelope too far so the stories got out.
The massive emphasis on climate doom is discouraging many young adults from parenthood. Many of them believe it would be irresponsible of them to "inflict another human on the world", and/or inflict the "doomed" world on another child.
While out country might be not currently at replacement levels, last I knew the world was still at more than replacement levels. More people are starting to think globally, not locally. At that point, it's a matter of opinion as to whether or not this is a bad thing.
Society has made marriage an undesirable burden. You're an independent woman, you don't need no man right? And sex is easy so who needs a wife. Many consider the odds of divorce and the consequences of it and feel a lack of incentive to marry.
I'm going to disagree with this. I think what we are seeing is a decline in the legal marriages yes, but an increase in many of the marriages such as Heinlein had in his stories. Looking at
Time Enough for Love, we have the story where he is on the frontier planet, and he ends up marrying the (no longer) little girl he rescued from the house fire. Their marriage was nothing more than simply declaring they were married. From the descriptions, it also seemed to be that many of the marriages on Tercius (sp?) were also of this type. So is marriage an undesirable burden, or is it just taking form other than legal? We have people out there who have been living together, and having children, for decades without ever getting the legal status.
Women of prime child-bearing years are encouraged to pursue career before family, and are accepted in combat units in the military. Whatever you may think about this personally, it doesn't help the starting premise.
Given that Heinlein often put his women characters in the military or in positions of combat, and willingly so, I don't think he would see this as a problem. Further, I don't recall him writing such that one took priority over the other.
There's the "incel" phenomenon... some say because 80% of young women are only interested in the top 5% of men, and ignore any man who doesn't have a movie-star face and abs, plus style, verve and of course, money to burn.
I think the incel phenomenon is more due to the attitude of the incel, than that of the women. Not denying the shallow women that are out there, but even us nerds/geeks were getting a decent amount (once past high school) because women we seeing that they were treated better by them than by the men who thought they were God's gift to women. Women started placing a priority on what they wanted instead of who they could get. And sadly, what gets the attention are the shallow ones who are picky and want to have their cake and eat it too.