As the BLM, and I assume the more experienced between you two, I think you're more in the wrong in this matter.

Asking something unexpected shouldn't draw immediate indignation and refusal. Asking a DPS to mark is rare, but it is certainly something you are able to do.

SV can be rough on a newbie tank and sleep can help a lot. I've recently re-learned the value of repose as a WHM in lowbie dungeons where inexperienced tanks simply cannot hold threat on all the mobs at once when healing needs are high. In a roundabout way, that tank probably had enough on his plate, was feeling stressed, and was hoping to delegate away some responsibility. Not doing so made his job harder. I know when I'm tanking and my party is not cooperating with me, I don't feel terribly motivated to give my full effort either.