Basically he meant that a man who would live their whole lives in a small town somewhere in Wyoming, for example, would never really had the chance to meet someone who felt the same way they did or reciprocated their feelings. They'd stay and get married to a woman they weren't interested in and that would be that. But the War meant that guy from Wyoming, and thousands of others like him, were transported to someplace like San Francisco or NYC. Where he would meet more men or women that felt the same way and have a chance at forming a community. Is this accurate or bullshit?
It sounds like your professor is invoking some of what Margot Canaday discussed in Straight State: Sexuality and Citizenship in Twentieth Century America, where she placed the military at the center of creating the legal and social definitions of homosexuality. As you/they mentioned, the war brought people (men AND women!) together outside the purvey of the normal heterosexual mixing they would have expected in peacetime. When the military sought to penalize or criminalize people who engaged in homosexual behavior, it had to create a new label, and legally define what that label meant. Thus, any person prosecuted for homosexual activity, no matter how incidental or occasional or experimental, became "gay," and were, among other things, dishonorably discharged and barred from receiving GI benefits.
Piggyback followup: how much did homosexual relationships in the military impact public perception of homosexuality during WWII, if at all.
I think your professor here is invoking Allan Bérubé's Coming Out Under Fire: A History of Gay Men and Women in World War Two, first published in 1990. As he puts it in his introduction:
Begun as the story of how gay Americans had served their country, this book evolved into the history of how the military’s mobilization for war made soldiers confront homosexuality in their personal lives and changed the ways that homosexuality fit into American institutions.
He provides numerous examples of soldiers who fit the mold of your hypothetical guy from Wyoming.