Ugh. Yes. I know this one well.
Person: So, do you worry that your boyfriend is cheating on you?
Me: It's crossed my mind that it might be a lot easier to cheat when we're long distance, but no I don't really think he is. I've been faithful, so I'm sure he's capable of doing the same.
Person: But you don't really KNOW do you?
Me: In the same that you don't really KNOW that your SO isn't cheating on you either.

Person: He's probably cheating on you though. You know that, right?
This is usually coming from people who I know have cheated on their present or past significant others, so what I've been tempted to say is "just because you cheat doesn't mean everyone else does."
Ultimately, I trust the BF and I don't think he's cheated, but my constantly worrying about this isn't going to stop him from cheating, so what's the point? I'm going to trust until I've been given a reason not to.
People can be seriously rude with their questions and comments. The one that has been bothering me lately is when people try to give me immigration advice because they know I'm planning to move to the UK next year. Most of the advice is flat out wrong and when I point out the flaws in their statements they're like "no, no, no. That's definitely how it works." Yeah, ok.
