Another Stoormtroopers are given suppressants. So when Finn defects he's no longer on them. Finn turns out to be an omega, who is in a relationship with his Alphas Poe and Rey. Since he's been on suppressants his whole life, he hasn't had a heat with them yet. He gets recaptured by the First Order, specifically by Alpha Kylo Ren. Finn's heat hits before Poe and Rey can break him out, so he ends up spending it with Kylo Ren.

+1 Kylo Ren not realizing Finn's an omega at first, and this being Kylo's first time with an omega.
+2 Grandma!Leia doting on Finn because he's carrying her grandchild.
+3 Poe and Rey treating the unborn child as their own.