(Welp, may as well do this myself)
To my understanding, it is offensive to refer to trans people by their birth genders. This birth gender is usually the same as their sex. By basing sexuality on sex, rather than gender, you are by implication referring to their birth genders (most of the time; sex has its edge cases) instead of their current gender, so that is the part that would be dangerous.
Of course, basing sexuality on sex has its merits for enbies, (since their genders can vary even more wildly) but some of them don't like their birth genders that much. I'm somewhat in that camp. That's why I prefer the absolute terms of androphilia, gynephilia and ambiphilia for myself because they make no reference to gender, nor sex. They just are.
There's edge cases if you try defining it by gender, and a different set of edge cases defining it by sex. It can become nonsense for enbies defining it by gender, and it's offensive for binary trans people defining it by sex. Neither of these cases are perfect, but at least defining it by gender is better than the alternative. Personally, I'd follow specifically what a person says about their own sexuality, because labels should come from the person themselves, not you as a third-party.
I suppose you could change sexes, but last I checked, that's not fully realizable yet. A trans woman who undergoes a sex change operation isn't capable of pregnancy, I think, but my knowledge on this isn't very good. The important thing is that it helps the gender dysphoria that these people often face, but it isn't necessary to have a sex change (hormones, surgery, or others) to be trans. Some places don't let you have them, and the places that do often incur a massive cost in waiting time, money, or both.
Hell, the notion of a "biological" sex isn't even simple. Your chromosomes could be XX or XY, your hormones could be pointing in male, female, or nonbinary, your body could be male, female, or NB (because the cells sometimes don't listen properly to the hormones), your genitals could be male, female, or both, and your mind could also be male, female or NB. Cycle through the possibilities, and you'll notice that it's all edge cases upon edge cases the closer you look. I'm not even listing every possibility. Nothing is simple in this world.