I am not so sure about that. I think in a lot of cases it's a fabrication the left tells itself rather than understanding why conservatives tend to push back less on fairly conventional arrangements in marriages.
That's not the same as saying that men and women need to fulfill conventional roles. There are plenty of conservative women with careers and who aren't slaving away in the kitchen. And conservative men who help out at home or are very involved with their kids.
(My grandfather, who was very conservative and in many ways a very manly man is a good example. He was the one (a long with the sons) who did the dishes in that house. He also washed the kitchen floor once a week, because he didn't like seeing my grandmother working on her hands and knees. In the evening they'd watch TV and he would always make up a tray with their tea and biscuits and bring it down for her as well. Now lots of men don't do these things, but that isn't a right/left wing thing. It's because a lot of people don't care to put themselves out if they can get away with it.)
One of the things I think a lot of people on the left miss about the conservative approach to gender roles though is that they tend to place a higher value on some traditionally female roles - particularly the role of the mother and homemaking - than people on the left. Not just valuable, but something that it takes time and commitment and isn't just an afterthought. Many on the left see these as things that are not only not so important as a job would be, but they see them as demeaning, unless people are being paid (but even then they tend to be wc people, so there can be a snob element, anyone can do this work.)
What that means is that a woman staying home with her young kids in a conservative setting isn't seen as less valuable or worthy than her having a paid position - and so if she would like to do that, it's seen as the responsibility of the father/husband to facilitate that by earning a good income. And women who come from a background like that and who have kids don't feel obligated to have a career or job unless they really want to or need to financially.
As for motherhood, a conservative would likely say that's not a "gender" role, but that being the mother (biological) has some different elements than being a father, especially in the early years when pregnancy and childbirth and breastfeeding factor into the equation. So they don't see that as imposing some kind of sexist arbitrary role but as a recognition and accommodation of differernt basic biological realities in reproduction.