Because of HOW they came to be together.
This is a very big deal.
What?? They were already falling in love BEFORE he told Belle about Nealfire. She knew he had a son, but never broached the subject until it was better to do so. You can tell they were in love during Lacey, Skin Deep (prior to discussion of Baelfire) and the S3 finale. They did not come together because of Baelfire. Rumple barely even discusses his son at all. He never brings him up to Belle–it’s Belle who brings him up.
GOLD DOES NOT WANT TO BE THERE. He wants his SON to be there. It’t a big deal.
Of course he wants Nealfire alive. He’s a parent. But if Rumple wanted to die, he’d kill himself. Simple. He killed Zelena to exact revenge and is now married and “moving on” because he feels that he defended his son. I have been very loud in my anger over this, but I *get* why Rumple killed Zelena. But Rumple is not suicidal. People who are suicidal do not get married.