"In reply Jesus said to them: 'You are mistaken, because you know neither the scriptures nor the power of God; for in the resurrection neither do men marry nor are women given in marriage, but are as angels in heaven.'"
And the ANSWER IS: The angels in heaven don't bear children and thus have the same "sex", they are androgynous, a combination of male and female. For the purpose of procreation until the earth was populated, the two sexes were necessary, male and female. Along with that, because of the necessity of family and raising children, marriage was instituted. But once you reach population zero and there is no need for children and that type of family, then marriage will be abolished. By the time of the second resurrection there will be no more births, the last human eligible for life will have been born. This second resurrection is after the millennium is over. There is no resurrection during the millennium!
At any rate, those coming back in the second resurrection will be like the angels in that not only will they not marry but they will be androgynous as the angels, being some form of unisex or bisex. With no marriage per se and everyone the same sex, and thus no suppression of women, everyone will be equal, intimate "sexual" (or whatever) expression of affection will be open to any two individuals if they choose. Thus it should especially be appreciated that angels have a concept of sexuality among themselves. They could materialize if they wanted and had sex with each other, there was no need to iimpregnate humans. Thus this was a direct, conscious rebellion against God. The angels were not sexually deprived, they were just bored and/or greedy and rebellious.
On a biological note, every human cell starts out asexual. Hormones cause the development in one direction or the other of male or female.
Of course, the WTS is no where near understanding this.
As far as "heaven" is concerned, only the Bride Class represented by 144K, probably the actual number is x10 (1,440,000), will go to heaven. This takes place after the final judgment which follows the millennium and Satan's last rebellion and destruction. So being like angels does not mean they will be literal angels, simply have a similar "sexuality" as the angels which does not require marriage.
JC