That is that they are too difficult, and lack a difficulty slider.
Some people want to add a lot of features to their game, make things interesting, and provide a little bit of challenge when it comes to playing. That is all well and good... except, often enough, they make it too much of a challenge, and end up creating games with steep learning curves. Now, this wouldn't be a problem for some, those who enjoy the challenge... but the fact that the game has porn in it (usually revolving around it), this typically means a larger amount of people only want a casual experience, something mainly to fap to.
So when the game becomes difficult, sometimes frustratingly so, the casual fap gamer is punished for their inability to progress past certain areas or enemies. Games like Forest Monster Victims and Medusa (with the unbeatable witch) can be annoyingly challenging for the horny. At least in those games, one simply has the option of cheating their way by clicking the hidden gallery buttons, so one doesn't have to fight the girls to get to the juicy bits.
A less pleasant example would be "Clixsposing Samus"... a truly shit game, it had 3 difficulty levels, and even the first was no cakewalk. And it was only upon completion of the infuriatingly hard third stage that the actual sex would occur... and what underwhelming porn animation it was. The payoff was negligible, leaving the gamer either angry and empty, or off fapping to something like Hentai Key Girl 5.
And then, there are certain other games which are great in execution and sex scenes... but have elements that can render the player rage quitting out of frustration and a lost boner (if they are male). Sakyubasu no Tatakai comes to mind when I think of this... GoRepeat had a pretty spot-on idea when he made this game, and the scenes were rewarding in their juiciness... but the game, for someone like me, was truly agitating in certain areas. It's biggest flaw was a lack of difficulty settings (something which Dragon Bride did quite well, with having an invincible mode and all), making some enemies truly frustrating; the triple wasps come to mind quite quickly. The addition of a difficulty level setting would've made the game far more enjoyable for me, the casual player who just wants to treat themselves.
It is perfectly valid to have a degree of difficulty in games, and I can see why designers would want it; it makes it more engaging on a different level... however, it is often at the expense of a casual fap gamers experience. High difficulty is often a flaw in the medium of flash porn games; there are not many people who actually want to work for their porn. Quite understandable. Those that do more often than not desire such for a more gaming challenge-related reason, rather than the labour of reaching the hentai. But a lack of a difficulty slider to allow for a much more casual, easy experience often degrades the experience for a good deal of people, having the frustration of beating a certain enemy overwhelm the payoff when the hentai is achieved. And when the hentai itself is poor, then there are generally bitter feelings all round... one doesn't want to work for their porn. For most, working for porn is working in the real world and using some of the leftover money after life expenses to pay for subscriptions.
So, in conclusion, I think any porn game that has a focus on challenge should also take into account that it is about porn; therefore, they should also possess the option of letting people breeze through it if they so wish; if this leaves the player feeling as though it were too easy, then they can simply up the difficulty level and play through again. If a designer doesn't want to make a difficulty level setting, or is unable to, then having a 'cheat' mechanism (ala Medusa and Forest Monster Victims) to simply access a gallery of scenes would suffice equally well. After all, porn is titillation, and should not induce rage quitting... that is simply a flaw on the designers part.