That's not entirely true though, just like with many f2p games out there, in both cases you can get and play the game for free. The subscription model allows for continued support to help. In the case of porn games, it's to help the dev hopefully free up time which hopefully then is used to work on the game. In the case of f2p games you are hoping they use the money to create new content and pay for servers/new servers, maintenance and keep the game running.
In both cases you are not investing on a business level and in terms of promised returns they are the same in that there is no guarantee from either that by giving (subscribing) money, that they will guarantee new content. While some few dev's offer extras, story, renders, streaming feeds etc. and some f2p offer quicker access to gear or utilities etc. (not including scummy pay2win) in most cases both subscribers and free players see the same returns.
In these cases people are looking to support the dev or game company, not invest in it. I think the confusion comes from the difference between a personal, emotional investment and a financial, business investment and mixing the two up.
With porn game dev's, the subscription model allows for easy of use, in that people do not have to manually donate each month and the tier system allows for ease of use, in that those that would like to support but can afford $10 a month can still give $1 a month. These systems just make supporting easier.
The biggest difference though is that if you invest in a company and then pull that investment out you will not see any returns again (unless you reinvest). If you donate / subscribe to a game dev / company and then stop donating / subscribing, you still have access to the game (although not the extras in some cases).