To be fair,
@Markness, you already said you didn't do the things, it's not an assumption that you wouldn't, you mentioned that you hadn't.
And the bit you might not be getting, is that each of us is responsible for doing our tasks, not for meaning to do them if reminded, we have to work out how to be there to do them at the required time. It's not unreasonable to expect this of an adult in the household.
Again, if you can't think of ways to ensure you can get your tasks done, it can be hard for the others at home, whether they are parents or partners.
Lots of responders here have suggested strategies or ways to approach avoiding difficulties of missing doing tasks and therefore putting more work on the stepdad. Maybe read what you told us again, and see better why we answered the way we have. People are trying to help, but it's hard for you to see that I think.