Personally, I'm leaning towards using the GUCK rules altogether for their simplicity, just modifying the BoEF feats (if anyone takes them) to be a relatively shorter time period per boost - 5 minutes per potential +2 seems about the same time-scale for GUCK standards.
Actually, I just spent a lot of time reading the GUCK to make Jakk and it looks like they really did simplify their sex mechanic quite a bit. Haven't worked out the BoEF mechanics for fuckin' so I don't know if I prefer it, but it's not hard.
During sex, rounds are 1 minute. Every round both parties will make two checks.
The first check sets the Arousal DC. Roll a Sexual Prowess Check. There's a table in the book you compare your results to which gives a modifier. The Arousal DC for that character in that minute is 10 + That Modifier + Any Tricks Or Feats that apply. For example, let's say Jakk rolls a 10 with his Sexual Prowess of +17. He would then have 10 + The Modifier being 8 in this case + 12 for having Enduring Lover 3 times which gives 2 each, Endurance which gives 4, and Iron Will which gives 2. DC 30.
The second check is your character's attempt to hold back their orgasm. Roll a straight Sexual Prowess Check. If they fail to meet the DC set by their partner they move up one sexual condition. If they were normal they become Horny, if they were Horny they become Aroused, if they were Aroused they become Peaked (or Over-Stimulated), and if they were Peaked (or Over-Stimulated) they have an Orgasm. Most of the time people start at Aroused in a sexual encounter. When a woman has an Orgasm she goes back to Aroused- or Peaked if she failed the Sexual Prowess check by 10 or more. When a man has an Orgasm he going down to normal or Horny and can't get to Aroused until a certain amount of time has passed, usually 30 minutes for most men but feats and tricks can change this.
As for how long sex can last, watch the Con scores. At the character's Con score they need to start making Fortitude saves every minute. The DC is 10 + 2 for every previous orgasm the character had that day. Every round after that the DC goes up by one. First failure means the character is Sexual Fatigued, Second Failure means the characters is Exhausted, Third Failure means they pass out.
I would suggest a couple house rules to clean this system up a bit. Women don't Pass Out until after either they have another Orgasm or their partner stops stimulating them, Men automatically Orgasm before becoming sexually fatigued.
So really, I think it's rather simple. Was considering writing up an example encounter to show this. ;P
I think you've got the first step mixed up a bit...the Arousal DC is set at 10+the bonus granted by the partner's Sexual Prowess check, assuming no modifiers from sex tricks. Endurance, Iron Will and Enduring Lover add the character's bonus to the save, not to the DC. From what I read, it goes like this:
1. Jakk goes to seduce Jill. He can become aroused as a free action, but she isn't ready yet. He whispers sweet nothings in her ear, getting a 22 on his Diplomacy check, and she fails her Will save against becoming aroused.
2. They get down and dirty, Jakk makes a Sexual Prowess check and gets a respectable 25. This adds a +8, so Jill's Constitution check DC is 18 vs. becoming Peaked. Jill makes her own check and gets a 22, making Jakk's Constitution check DC a 16. He has Enduring Lover, Iron Will, and Endurance, though, so he gets a +8 bonus on his roll to resist becoming Peaked (Enduring Lover can't be taken multiple times, unless I missed something).
3. If either fails the check, they become Peaked. Repeat step 2 until one or both of them fails two Arousal saves in a row, at which point they orgasm. After Jill orgasms, she is Aroused for 1d4 minutes unless she failed the save by 10 or more, in which case she is still Peaked. After Jakk orgasms, he is Fatigued for 2d6 minutes.