I do not pass out, but I have occasional post-orgasm headaches, and it helps me to drink tea before doing anything. Pre-hydrating helps avoid headaches - I just learned that and I am pretty happy about that. It is an easy fix. Maybe you also need a complete snack (protein+complex carbs+some fat, say PB/whole grain crackers/some fruit) before sex or in-between orgasms. At least it would not hurt to try. I have been known to pass out in non-sexual contexts, in the morning before having breakfast, and having breakfast first thing in the morning resolved that.

I get hungry, at times, from partner sex but not from masturbation. Weird.

But basically, if you pass out, you should first try proper hydration and avoiding hunger, and see if it helps. This is just the general rule against passing out in non-sexual situations.

You can look up "orthostatic hypotension" and "vasovagal syncope" to see if you have had those.