That door swings both ways, Baby. I made a vow to never even date, much less marry, a MAN with children. As you said, “the thankless task of raising another” woman’s children.
I don’t blame you a bit for it - particularly if you’re not bringing kids of your own into the relationship. That strikes me as not only reasonable but completely fair to insist on the same in return for anybody you would consider getting into a relationship with.