About SY and MJ having children - I'm pretty sure they can have them. MJ would have told SY if it were impossible. He wouldn’t withhold such crutial information from her when he knows how much she desires children and we know he wants them too, for God’s sake, he wants 7 of them! He would have told her during that conversation if they couldn’t and he already knew back then that he will try to find a way not to disappear. MJ wouldn’t have concealed the truth from her no matter how cruel it would be as they have always been brutally honest with each other and he already told her all his secrets.
Remember he is an alien explorer and scientist so I bet he looked into the possibility during his years on Earth. Plus he has a decree of medicine, and maybe multiple of them, and I wager he is well-versed in genetics. As I can remember MJ only said in the first episode that he can’t mix saliva with humans, he didn’t mentioned other bodily fluids!. Blood is obvious since he doesn’t have the same genes as humans and he would be poisoned by their blood just like a person with AB+ would be if injected with blood type A-. And even if he couldn’t mix his own with human fluids, when MJ came back his abilities were stronger than never (more complex and longer time stopping and ability to teleport himself to location in far away Galaxy) and most importantly he didn’t collapse after the red carpet kiss! but instead teleported himself immediately which had been impossible after all the previous kisses when he always fainted few seconds after them. SO it seems clear that when MJ re-tanked his mojo at KMT 184.05 he developed, together with his upgraded abilities, an immunity to it and so overcame his very inconvenient allergy. YAY.
About not showing them having 7 children or any children at all - maybe we’ll get them in the DC (wrote this before the second epilogue). I mean they need scenes to add there plus there is the matter of the new shortened episode length from 65 minutes to 58 so I bet the PD couldn’t include all the scenes he wanted. While I would love to watch MJ and SY try to bring up 7 half-aliens helions I don’t actually need to see it with my own eyes because I firmly believe they will have them all, what’s more I wager to say there is already one or two of them in SY’s belly. I mean there has barely been any night they haven’t slept with each other.