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Everything I've read so far has been from a very n0vice perspective..
I shall leave some exp here for anyone that may need it..
At some point humanity produced enough food so that we no longer gave a damn about eating. It was demoted from a desirably thankful reward to, "yea, I'll get to it, no time, Top-Ramen."
Then... humanity produced so many PEOPLE, that intercourse was no longer a absolute function. It too was demoted from a necessity down to, "yea, I'll get to it, no time, pr0nhub."
---the OP himself demonstrated ^^^ this point perfectly..
So my point is.. with every pleasurable or necessary excursion there is a TIME COST..
Once that cost becomes extremely LOW.. there is no longer a need to attach so much emotional oversight upon how and when to do it..
There is no reason to prepare complicated food dishes, when simple foods are of equal quality and nourishment...
There is no reason to Romanticize SEX, when in fact our species is Extremely Overpopulated...
The -Imperative- is GONE..
So what is Par value...
It used to be the case that the male must have certain resources in place prior to acceptance by a female.. This was also very important because the GOAL was important, procreation and rearing..
NOT SO any more.. The GOAL has been reached by humanity as a whole.. a sustaining population relative to the immediate environment
Which means, the average female and male are now equivalent in their sexual exchange.. since the baby is not an absolute consequence..
Bonding dynamics:
If the female has not fully and repetitively entered full engagement with prior mate.. they are essentially NOT chemically bonded.. or at least the bond may not be strong.
Oxytocin is significantly stronger in females, up to 4x, than in males..
Which means.. we can crudely say it's much harder for a female to suggest breakup than male.. this is an evolutionary strategy built into female humans, as they need resources during child rearing that is optimally provided by a 3rd party, so together they can achieve higher economies of scale.
During a breakup, the female is also more likely to engage in "rebound" behavior, because she is experiencing a lapse and a "greater emptiness" than the male...
Whereas the Male can just drink it off... and feel great that very next day....because he has time for m0re d0ta and no one to tell him there's a regulation amount of cuddling per day...
Now.. towards the VERY specific trade in the OP..
Assuming the two parties involved are both using new cars of equivalent value.. let's say a camry for an accord.. same freakin car.. no miles on either...
It's really a 1:1 trade..
Here's the problem.. if you're both 19, i'm assuming college..
If the prior buyer for that female goes to the same school as the two of you, then you must check if this female is a "discreet" seller..
It's not worth fighting over girls, there are so many alternatives in college, you'd be stupid for taking a punch or making enemies with a guy (and his possi, frat, guild,etc)...
if the girl is NOT discreet, only engage if the guy went to a different school, in which case the distance is generally too costly for the prior buyer to seek vengeance (comeback for the car)..