Munchies:
It's about boundaries. When someone has a boundary, you do not cross it. Now, in some contexts, if the relationship allows for such closeness, you can have a conversation about that. But seeing as OP has been dating his gf for 4 months, he ain't there.
Enas:
I wrote this mostly as a reply to Milhause said, which can be better described as some sort of silent social pact between people rather than everyone's boundaries.
As for the OP, it is a metaphorical jungle that they have to go through. It seems their relationship has not been solidified adequetally. (that will be true ehrn they can talk about anything without trying to dodge topics)
Beloved, the silent social pact between people
is a boundary. You know why women don't want to talk about such things. You admitted as such yourself. Therefore, talking about it without expressed permission from that person it crossing a boundary.
It's like if I decided to ask you about your finances. Unless we have a relationship where it's appropriate to discuss such things, it's terribly rude and invasive. As such, there's a silent social pact not to do that.