Oh, I thought you were trying to prove from the donkey halacha that a person is obligated to accept a chessed. That was the part which I thought needed proof.
In any event, I think you are looking at things too black on white. Tznius is not binary; things are not right or wrong; there are balances and nuances.
I think that it is appropriate for a man to offer to help a woman carry a stroller, since she is the one who needs the chessed done and he should not be frum on her cheshbon. But, I think the appropriate thing in this situation is for her to decide whether she absolutely needs the help and otherwise to decline.