From Gotquestions.org:
"The Bible never directly says whether angels reproduce, but Jesus touched on a related subject. When asked about human marriage in eternity, Jesus replied, “At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven” (Matthew 22:30). The fact that there is no marriage among angels has led some to believe that angels are “sexless” or genderless. Of course, if angels are genderless, then a safe assumption is that they do not reproduce—but that conclusion cannot be proved from the text. The fact that there is no marriage among angels does not necessarily mean there is no gender and no procreation. Angels do not marry, but we can’t make the leap from “no marriage” to “no gender” or “no reproduction,” however logical such a leap would seem."
Are you, and GotQuestions.org, suggesting that God changes His stance on sex outside of marriage when we get to Heaven?
Generally i agree with you, but would add the following, For the birth of Jesus, Mary a mortal was overshadowed by the Holy Spirit Luke 1:35 ,- The result was the man/God Jesus. From this and Matt 22:30 my personal belief is that there is no marriage and no sex in Heaven. I account for the offspring of fallen angels as follows. We know that humans can be possessed by demons, and I consider that if a person has caused/ allowed themselves to be possessed, then a demon is able to manipulate that persons genetic DNA chain to produce offspring. In a slightly similar way as was Jesus birth. Personally i think this is the only way a demon can reproduce. But again as you have rightly pointed out, we need to be careful to avoid being dogmatic when there is very little Scripture proof to go on. We will get our true answer in Heaven. Our present beliefs regarding this will not affect our salvation. Better to get on with ways to reach the lost.