Marco groans in frustration and drops the attempt with his pheromones to encourage it. But it's too late; it's a hormonal runaway of mounting proportions. His body's reacting the way a beta of sufficient age would to an omega pouring out these hormones - priming the need to take the role of an alpha.

It won't magically make him one. But his body has the capacity to knot just like one, with the right trigger.

He raises a hand to wipe at his mouth - seriously, why is he nearly drooling so much all of a sudden? and speaks again.

"... I stopped. Why aren't you stopping too?"

This isn't accusatory at all. Now he sounds worried, especially with how Jean is curled in on himself and hiding his face in Marco's thigh. He mentioned something that Marco almost missed in his panic. Cramps?

Oh. Oh no, cramps.

Marco timidly reaches down to brush some of Jean's hair back, and though he knows it's risky he remembers what they taught him - how to check if a gland is swollen. The instant his fingers brush one of Jean's, on the side of his jaw exposed to Marco, he knows.