As a family, money needs to be earned, and the children need to be looked after. There are many ways of arranging things so that both are covered. If you and your partner, as equals, sat down and worked out a solution which suits you both, and in which you are both still equal, then fair enough, whether that means you are at home, or he is, or childcare is paid for.
The problems arise when staying at home is seen as the lesser of the roles. When people say a SAHM is dependant on her partner, it does not occur to them to think that her partner is dependant on her for childcare.
So, in a partnership of equals, staying at home is fine from a feminist viewpoint. However, assuming that you should, because you are female, is not. There are also issues with the way society is structured, as mentioned by others, which make it difficult for a couple to operate freely as equals, such as rights to time off and unequal pay. If a man can earn more, and a woman can legally take more time off at the beginning, this places outside pressures on the choices which are practical, pushing people into traditional roles they may not otherwise choose.