Do couples have to share e-mail passwords?

Recently, my colleague and I got into trouble. We chatted on Facebook and apparently, the read was saved in her private messages. We were gushing about the guys in our hospital whom we have crushes on.
My colleague has a boyfriend who also work in our institution. I did not know that he knows her password to her Facebook account. I just received a test from her that her boyfriend opened her account and read our exchanges of girl talk.
According to her, her boyfriend was so furious about the matter and I asked her why on earth did she let him access her Facebook account. I told her that it is not a matter of trust but it is just a matter of personal space.
They ended up patching things up but he guy does not talk to me. As if I made the sole act of luring his girlfriend in “looking” at other guys.
For me, I do not see the good side of letting someone break into your personal space. Just because your significant other knows your password to all of his or her account, it does not mean that you cannot hide anything from him or her. There are ways.
Just sayin'.