Ok so I can see how this could be a bit confusing. I changed some of these rules last minute so the formatting of the pages was already done, limiting the space I had to go into extra detail.
Firstly the Sex(dex) rolls are just to determine how good you are at pleasing your partner. So basically how good you are at sex in this particular encounter. This is more relevant in a roleplaying sense and less in a mechanical one. For instance if you are sleeping with someone to gain information then better sex will mean more information etc.
(Originally I wanted the Sex(dex) roll to affect your partners Composure(con) rolls, adding or subtracting time. But I couldn't find a way to do it that wasn't needlessly complicated)
Next your Composure(con) roll determines how long you can last before climax. Males have to wait after climax before they can get aroused again (10-15 minutes) but females can continue the sex immediately after their first climax. So if the male finishes first, it is likely that the female will not get to climax at all. Males can continue to pleasure their partner with their hands or toys after climax but they cannot gain arousal during this time.
If a female climaxes before their partner they can roll Composure(con) again to see if they can climax again. (though I didn't write it down in the book I would probably make subsequent composure rolls at disadvantage if you already climaxed during that sexual encounter)
And it should be noted that these mechanics are specific to sex outside of combat. Combat sex has a completely different system of rules. So stuff like arousal points don't really come up in regular sex. I hope this all clarifies things.