It's extremely easy not to assume someone is straight, I promise you.

I get that you're saying you can't just never assume anything about anyone, and it's impossible to never ask a question that really doesn't apply to someone due to unusual life circumstances.

But there are plenty of gay people. Like, PLENTY of them. And my gripe with this assumption is that I have to deal with it literally EVERY TIME I meet someone who is going to stick around for any length of time (eg coworkers as I mentioned). It's as easy as asking instead of "do you have a boyfriend?" something like "are you seeing someone?" or similar. Boom, you have not assumed. Maybe in your heart you still kind of assume they're straight because most people you know ARE straight and some % of them are gay but you had no idea because you assumed and they didn't correct you... but it's your actions that matter.

I think a lot of your examples are false equivalences, anyway.

"Do you plan on having kids?" is not necessarily a bad question to ask to someone who doesn't, in fact, plan on it, because they can in fact just say "no, I don't plan on it." However, ASSUMING that everyone wants kids could be really annoying to someone who has struggled all their life with such an assumption, such as just asking "so when are you and the husband popping a few out now that you've been married for 3 hours?!" and then getting all awkward and weird when they say no, they can't or don't want to.

Something like discussing what to do when retired or Christmas plans are incidental events. Sure maybe you could accidentally hit sore spots and there's no way to avoid that always. But being gay is a constant state of being that I feel like I am constantly either hiding or uncomfortably announcing. I mean, don't get me wrong. I am not saying someone is a bad human being or that I judge them for assuming, and I especially let it slide if someone is older, from a different culture etc (still don't enjoy the process but IMO they can plead ignorance). It just makes my life a lot harder, makes you look less well-informed than someone who DOESN'T assume, and it's super easy not to assume if you are remotely educated on the topic.