I don't agree with the grandfather pressuring marriage but I understand his feelings; he is lonely and is afraid Huan Wen will be, too. He genuinely wants his grandson to have the full emotional experience of having family connections that he wasn't able to experience as a child.
I might be wrong, but seeing how much the grandfather cares about Huan Wen, honestly I don't think he would reject him if he comes out as gay. If Huan Wen finds a partner, regardless of gender who can love and accompany him throughout life, then I think the grandpa will feel at peace.