If you met one of them, how would you "know" the difference between a woman who had sex with you because she consented Vs because she wants to hurt herself through sex? Unless you are able to read minds, how are you going to know you didn't just victimize a woman yourself?
Communication and social skills mostly. Read body language and the general context of the situation and how it lead up to there. Some may think that "well I shouldn't have to do that", I believe it's necessary when being an empathetic, semi decent human who sees the person they are going to engage in intimacy with as a complex being with feelings rather then an object.
And if that isn't possible, for whatever reason, or you remain insure of the person's consent you can just walk away.
Also, I think this is a particularly complicated example. Because for example, people can be lead to join kink communities to find other like-minded people to play out such desires, and all that under usually very long, detailed and extensive discussion about boundaries and consent within what they decide will be a safe space. (When kink is done properly obviously...). But also, if a person wishes to engage in intimacy with the intention of self harm, are they able to give consent, is their state of mind or struggles not clouding their judgement as one would have if under the influence of substance.
And that on some level, that kind of makes the sex non-consensual (by your definitions) since it came as a result of a transaction where the man paid for a date and the woman felt on some level obligated to give that man sex as a result?
Obviously inviting someone out for a meal and evening, paying for everything does not make anyone entitled to perform any sexual act in return. You mentioned this above and I agree, it is wrong. I'd also add that yes, if a person feels pressured to repay through sex, that is a form of coercion, that is SA.
I mean, if you get right down to it, what even is "consent"?
"Consent is the act of voluntarily giving permission to a specific action. For consent to be valid, the individual must be fully informed of what they are consenting to, and give their consent without coercion."
Under the GDPR consent must be granular (= Granular means the component parts of a whole are easy to observe. For consent to be granular, the consenting party must be aware of each specific action they are consenting to.)
Consent requires voluntary, informed, mutual, honest, and verbal or non verbal agreement.
Consent is ongoing and must be asked for every step of the way of a intimate encounter, in any way the people involve choses.
Consent can be withdrawn at any time, and consenting to one intimate activity does not automatically mean consenting to another.
All this is why children cannot give their consent for example. And if you want to be daring and push it, animals too.
I mean, we are all made up of chemicals and biological urges that sometimes drive us to say and do things we otherwise wouldn't...
Humans have lost connection with most biological impluses for a long time now. We no longer have a mating cycle (estrus cycle). Animals operate on instinct, humans on the other hand usually act on executive function. Instinct is not determant, cognition is.
For example, some chose a life of complete celibacy. It can be because of religious beliefs, psychological hindrance, or simply the way their cognition is wired. Asexual and aromantic people for example. Humans can override reproductive impulses and have for decades, that proves that decision making in most cases overrides biological impulses. Also during intimacy, humans negociate and generally share preferences for intimacy, set boundaries and share past trauma at times. Animals who are closer to biological impluses don't.
I can go on.
so do any of us truly consent to sex with another person?
Yes. Absolutely.
Or are we driven by our body chemistry to "mate" beyond our ability to actually consent?
Consent exists because as humans we are capable of saying both "yes" and "no" from a position of mutual awareness. And either can be stated or withdrawn at any point in any intimate encounter. "Modern" humans like us, the term might not be correct, are not guided or blinded by mating impluses, but rather reflective decision making.