My idea is that there should be stamina but different positions are taking different amount of stamina in certain levels.
This is not how it is going to be, it is how i would like it to be:
So to not lose any stamina while giving a handjob, you have to be 5 levels less the enemy or higher
Blowjob the same as the enemy or higher
penetration 5 levels over the enemy or higer.
But each of the positions is also giving different amount of xp so you can choose between to lose more stamina and gain much xp (its a bit risky because theres a chance that you cant take it) or you gain a little of xp and not lose any stamina at all (it may be safe but you wont earn as much xp)
So you have to think before you do but you can always go back to some low leveled enemys and dont worrie about what you are doing but again you wont earn as much xp as you do it with higher levels. Your stamina is going to fill up by time so you can always have some fun.
But you wont wont lose easly.