Why do people feel the liberty to ask personal questions about my sexual life when I "come out" or let them know my sexual orientation?

They lose their filter a little bit. They become curious; you don't have the same experiences as them (necessarily), so they want to know the differences and similarities. It's a wee bit rude in most contexts, but usually it's curiosity mixed with loss of filter. Usually people aren't intending to be rude when they ask, nor do they aim to make you uncomfortable most of the time. Some do, but most just don't know how to filter some things out.

We live in an incredibly heteronormative society and people always feel the need to know everything about everyone. If someone isn't "straight" it's kinda seen as a big thing as the majority of the world is. Some people might just want to better understand someone's sexual orientation by asking questions. Generally, I've found that a lot of people want to know a persons sexual orientation to better judge what kind of person they are or kind of fit them into some type of queer stereotypes. Being queer isn't seen as being normal because of the amount of straight people. Many people are discriminatory towards queer people due to religious views or upbringing and so generally people want to know that kind of stuff.