Apparently he was the one who gave Tir the "birds and the bees" talk when he was younger. After Pahn said something he shouldn't have in front of the inquisitive young boy - and when asked what he meant, he stammered and turned red and said Gremio could explain it better. (Gremio: Thank you ever so much, Pahn. >_<)
If nothing else, though, Master Teo was very grateful to Gremio, because it meant he didn't have to have that talk with Tir. And you know, being one of the Great Generals of the Scarlet Moon Empire is no problem, but giving "that talk" to your son? Brave as Teo McDohl might have been, that was one conflict he'd happily have run away from.
(And it appears that Cleo had a fling with Alen, which is what accidentally started this whole unfortunate conversation. Heh heh heh... the McDohl household gets more and more amusing every time I let Gremio talk.)