It is absolutely fine to organise your life however you want to organise it, as long as it is your choice.
Now, often people (women) go back to work and don't have any choice in the matter - maybe their DH insists on it, maybe they economically don't have a choice. That would be of concern to feminists.
Often people (women) go back to work but still keep on the lion's share of everything else as well - the cooking, cleaning, childcare beyond their working hours, the negotiating and communicating with childcare, the drs appts - while their DH's life continues pretty much on the same groove as it always has. That would be of concern to feminists.
Often people (women) go back to work and find that, even though they are back in the workplace, they never make up that 'gap' of maternity leave, their experience is downplayed, they are overlooked, they don't get access to the same opportunities 'because you won't want to travel because you have a wee one'. That again would be of concern to feminists.
Or that even people (men) would love to work part-time or more flexibly, but their employers look askance and say 'haven't you got a wife to deal with that nonsense' and refuse the request. That too would be of concern to feminists.
And finally, often people (women) do SAHP but are looked down upon for it, they're seen as wasting their education and opportunities, they become financially vulnerable and five years later they find it impossible to re-enter the workplace, because after all, who wants to employ someone with a mummy-mush brain? And yep, that too is of concern to feminists 
You can honestly do what you like as far as feminism is concerned - as long as you are exercising your own right to choose and that society is not going to disadvantage you because of the choices you made.
(Although I'd be careful bracketing SAHP'ing as 'natural'. It's not what happens in many, many cultures and classes - working class women tended to work where I was from, for example and that was seen as 'natural'.)