Who should the role of breadwinner in a relationship go to?

Should it go to the male, female or does it simply not matter? Also if it goes to the opposite gender (i.e if male then female breadwinner) would it cause you any insecurity such as a lack of masculinity? Im am asking in terms of a heterosexual relationship.

The question I am asking does not pertain to the definition of breadwinner, but who in terms of gender should be the breadwinner in a relationship and why.