Very in control, such not being ruled by hormones, wow.

At the very least, Marco knows not to bite where it'll be dangerous, in case of starting a bond. Instead, he keeps kissing along Jean's neck, like Jean had done to him. Once he reaches the curve of his shoulder, though, he gives into the desire to bite and does so, sinking his teeth in. Not enough to break skin, but enough to leave some sharp indents, sucking at the skin between his teeth before he releases it.

Yeah, that's a decent enough mark that it settles the thing that had flared up in his brain after he got a potent dose of Jean's omega hormones.