Why did Stanley serve in the Engineer Corps during World War II instead of a regular combat unit? Well, a common stereotype of Poles is that they're good at mending things, and one of the roles of Army Engineers is to design and construct dams and flood prevention systems to prevent flash floods.
In 1947, while the term "straight" was not widely used as a synonym for "heterosexual" the way it is now, it was already being used that way within the gay community (early usages of the term date back as far as 1941), of which Tennessee Williams was a part. Taking that into account, it seems entirely likely that his usage of "straight" in the scene between Blanche and Mitch was no accident (especially given a certain line by Blanche that many LGBT people have found significance in in the decades since).
