Unlike BoneTown, there doesn't appear to be a "sex difficulty/challenge" for the different body types, but since there is no way to improve Fort Worth's "sexual prowess" like in BoneTown (inwhich you had to endeavor to increase your balls' size to even have a chance at pleasing the "hotter" chicks) I can see why this is not apparent in BoneCraft.

And while the EASY and NORMAL difficulties might not pose a problem, HARD mode might be a tad bit more frustrating in certain situations if it took a long time to please certain girls. For example, during Chapter 7 the objectives during each stage is to bone 10 elven chicks while being constantly attacked by the elven guys who continuously respawn. If the player took it upon themself to bone the elven chicks (not waiting for the other Wranglers to do it for them), if it took too long to actually please each of the chicks the player might find the rest of their Wrangler squad getting killed too often (they are not too good at getting rid of those elven archers). And as you have already stated, initiating sex can already take long enough, so add that with possibly getting interrupted while trying to initiate sex with the above scenario and it feels like it could get more frustrating than being a challenge.

But apart from what I've said below, I do agree with your thoughts on how the sex "mechanic" is handled in BoneCraft as opposed to BoneTown. Though with how BoneCraft plays with infinite enemies, squad/player health to keep track of, no real "sexual performance" upgrade system for the player, etc., I see it as an unfortunate balancing issue between being able to handle the real-time fighting and sex options while the fighting is going on.

I hope my opinion made sense.