I would like to suggest that you take the unpopular position that the American and Jig are married. Most people who have commented on this story believe they are just living together. I can offer a number of reasons to support the position that they are actually married.
The subject of marriage is never mentioned by either of them, although it seems unlikely that Jig would want to have a baby--in those days--without being married.
The American repeatedly tells Jig that she can have the baby if it is important to her, but he does not say that he will marry her. This strongly suggests to me that they are already married but he doesn't want a baby. Here are instances in which he tells her she can have the baby:
"Well," the man said, "if you don't want to you don't have to [i.e. have the abortion]. I wouldn't have you do it if you didn't want to. But I know it's perfectly simple."
"I don't want you to do it if you feel that way."
"I don't want you to do anything that you don't want to do--"
At one point the girl asks him:
"Doesn't it mean anything to you? We could get along."
This sounds like wife talking to her husband about their financial status. She thinks perhaps he is opposed to having a baby because of the expense. This may in fact be true.
"I don't want you to," he said. "I don't care anything about it."
He actually tells her five times that she can have the baby if she wants it, but never a word about their getting married. He could not ask her to have a baby without their being married. It was just unthinkable in the 1920s.
Their luggage is covered with stickers from all the hotels they have stayed at together. In those days it would have been quite difficult for unmarried people to share a room in a respectable hotel. They would actually have been turned away. In Europe travelers have to show their passports, which would prove they were not married.
He did not say anything but looked at the bags against the wall of the station. There were labels on them from all the hotels where they had spent nights.
Hemingway and his first wife Hadley actually did have a baby a short while before Hemiingway wrote this story. They nicknamed the baby Bumby. Hemingway was trying desperately to make a living as a freelance writer, and having a baby would have made his task nearly impossible. They were living in Europe because the exchange rate was extremely favorable to the American dollar, and Spain was the cheapest country of all. He would have had to take some kind of paying job, which might have necessitated moving back to America. This suggests that Hemingway might have been opposed to Hadley's having the baby but gave in.
Jig might have been less careless and more careful about avoiding pregnancy if she had not been married. She may have wanted to present her husband with a "fait accompli." Women does these kinds of things, don't they?
The only reason for supposing that the American and Jig are not married is that the author doesn't not say so. But this story is told in a purely objective fly-on-the-wall-viewpoint technique. The reader has to do a lot of guessing and make a lot of assumptions and deductions.
This is my suggestion for your argumentative essay about the relationship between the man and the woman.