It's Erin's load, that's why he needs to either cum into a condom, a bucket or lick it up off the ground whenever he's done being abused. Even during the super lovey dovey Erin x Mars scene Mars makes some comment about how Erin needs to either catch or eat any of his own sperm when they're done and that he'll explain why later.
Now Mars's load might also help sustain him but we haven't gotten there quite yet, probably soon after the time skip though.