一男半女 A Son or Half a Daughter
行者道：「師父不濟呀，那妖精安排筵宴，與你吃了成親哩。或生下一男半女，也是你和尚之後代，你愁怎的？」行者道：「师父不济呀，那妖精安排筵宴，与你吃了成亲哩。或生下一男半女，也是你和尚之后代，你愁怎的？」
逐句解 Line by line
Pilgrim said, "Master, you are in a poor way. That fiend is laying out a banquet, and when you have eaten it you are to be married."
He opens by stating the case flatly, which is either kindness or setting up a joke. It turns out to be the second. 不濟 is a vernacular no good, in a bad way, used of people and situations alike.
"And if a son or half a daughter should come of it, they would be your posterity too, monk. What have you to be miserable about?"
一男半女 — one boy or half a girl — is a set phrase for any offspring at all, and it is deadpan comic here. 後代 is descendants, which is exactly what a monk is not supposed to have. Pilgrim is teasing his master about breaking the first vow of his order.
Pilgrim's bedside manner is a running joke of the novel and it reaches something like a peak here. His master is imprisoned, terrified and about to be forced into a marriage, and Pilgrim's opening move is to congratulate him on the prospect of children.
The joke has an edge, though. It is designed to provoke, and it works: the next passage is Tripitaka grinding his teeth. Pilgrim has just asked the question he was worried about a moment ago — has the old man wavered — and he has asked it in the only way that would get a true answer.