
well, it's about age appropriateness.
lol at 'that might have been the wrong thing to say' 
you might know that what you mean is 'raising a baby as a single mother is hard and i wouldn't wish that for you - i want you achieve the world and not know hardship, and i want you to be hetero desperately because the world is a hard enough place without challenging the status quo', but an 8yo would just look at you as though you were a crazy lady if you came out with that.
if she was your 38yo single (i dunno, lawyer?) daughter and you were having the same discussion, it would be rather about what she was intending to do about work - was she going to take time out, or employ a nanny, or use childcare. and whether she thought she was doing the right thing/ was prepared for other people to judge her.
i tend to use the 'people usually are married when they have a baby, but not everyone is. sometimes it can be harder if you aren't', and then go on to discuss why if it comes up.
she just wants to know she can have/ adopt a baby without having a boring old man along for the ride.
she isn't really interested in the politics of marriage or single motherhood at this point. 
i think feminism has a lot to say about the institution of marriage... but i don't think that's what your 8yo means.
it is all about the expectations of women in our society - so is very tied up in the issues that feminists wrangle with, so not completley separate.
i prefer to think of it as not being narrow minded, rather than feminist in itself.