The song as a whole is written from the point of view of a woman who is having a relationship with another woman who, in turn, appears to be in an existing relationship with a man.
I'm breaking walls down
Is suggestive of an unapologetic stance on homosexual relationships. The term "wall" is a relatively common euphemism for a societal barrier rather than a literal one. For example, Cambridge dictionary has it:
a way of feeling or behaving that completely prevents two groups of people from communicating with or understanding each other:
There is a wall of mistrust between the two groups.
You can see a weak echo of this in the term "closet" used to denote hidden homosexual tendencies, not least since a closet is usually surrounded by walls. The walls being broken in the song are thus societal taboos around same-sex relationships that the singer is proud, "not sorry", to be challenging and breaking.
There's another parallel to this metaphor which is the term "homewrecker" to mean someone intent on breaking up a relationship which the singer is clearly doing. So she's "breaking walls" in more than one metaphorical sense, becoming a homewrecker. It is interesting to note that while the term is not gendered, it is far more often applied to promiscuous women than to men. So this also shines a light on the unequal expectations and morals placed on men and women's sexual behaviour.
In this sense the next line
Building your girl's second story
becomes a continuation of these two building metaphors but is also a play on words. "Story" can both a tale or a floor of a building (they're actually spelt differently in British English but the singer is American). The singer is "building" a new "story" for the object of her affections, a second one in which she is opened up to the possibility of homosexual relationships after her first, heterosexual story with her existing partner.
I'm not aware that
Ripping all your floors out
Has any particular sexual connotations. Rather it seems to be another play on words to continue the theme of demolition. Having the floor fall out from under someone is another common metaphor for a big shock or surprise, particularly in terms of important life events. The shock for the "boy" in this song, of course, is having his girlfriend leave him for another woman.
