It's not necessary to assume anyone's sexuality. If there's some reason you require the information, you could just ask. The problems with assuming heterosexuality seem obvious enough to me: it alienates LGBT+ people. I would expand on what that means, but I trust that it's fairly self explanatory. It would be irrational to assume that everyone was queer, it's perfectly rational to not assume anything.
I don't mean it in the sense that you walk past someone and explicitly think "that person is straight". It's more like if person A walks up to me and is like "hey, do you reckon person B is straight?" I would say "well I assume so, because most people are..." (this is assuming I otherwise know nothing about person B). This is the only rational assumption to make. Of course, one could choose to suspend any judgement, but this is of no use. I could just go and ask person B, but that is probably the most alienating thing one could do ("oh hey, are you gay?")
Personally I think the only un-alienating way to deal with this topic is to not let it have any significance for oneself. By explicitly recognising that others are of a different sexuality brings emphasis to the fact that they are different. It's better to say "ok so you're gay but it doesn't affect anything about our relationship", much in the same way it doesn't matter to me if someone I'm talking to is straight (using "relationship" in a general sense).
So perhaps one could say to person A "I dunno, who cares?". Or else "well, statistically one would expect they're straight, but what does that matter?" But then again, this is useless if person A wants to ask person B out.
Any assumption that someone who is female has a male partner or vice versa is just a subtle reminder that they are an "other" which is obviously isolating for people. It's nice to use language that is more inclusive for people and doesn't assume that they are straight. It's something small but can go a long way in letting people feel welcome. You could say "partner" or "spouse." Most people don't bother.
Ultimately one has to face up to reality. If I see a chick and I want to go ask her out, if she's a lesbian she isn't going to be interested anyway. And if some guy is interested in me he's wasting his time because I'm not gay. Acting as if LGBTIQs aren't different is completely impractical, because they are different and I'm not going to be all "yea I'll go out with you" to a guy, because I'm straight. And it would be really over the top for me to, every time I want to ask a chick out, go "hey I just need to make sure you aren't a lesbian, before I ask you out".
At the end of the day, everyone is different in some way. It sucks for LGBTIQs that they're different in a way which is generally extremely relevant to human social interaction (sexual relationships). But LGBTIQs only isolate themselves by acting out and making a big deal out of it.
Just as an example, I'm fairly short for a white guy. There is abundant scientific research showing that being short is a disadvantage for males in modern western society (ranging from all sorts of factors such as how competent people think you are, to the likelihood of getting a really good job, to your level of attractiveness, to seeing over the head of the tall dude in front of you...). And from the perspective of a short guy, if you look out for it it's actually really noticeable. But there's nothing I can do about it, so I just have to accept it and deal with it. That's life, and most of the time life sucks if you let it.
