This implies you can not tell the person’s gender, which for most people is perfectly obvious. So often can cause offence. I realize not using they/them can also cause offence, but just much less often.

This is so rubbish. Almost everyone is a he or she, so just use that. On the very very very rare instance you get it wrong, say sorry and use the correct one from then on. Unless you forget, the appologise again when corrected. Yes I am imply it is on the person who got called the wrong pronoun to correct the one who made the mistake.