I've got to say, as many others have before, this is the best integration of sexual content into a video game. I'm not sure how, but I think it has something to do with a "failure state" mentality a lot of other games have. Other games have the same "escape" function as Captivity, but it can be brutally difficult to get back up once you're knocked down. Captivity allows for much more of a chance to rally, assuming the enemy hasn't gotten to critical mass.
In addition, for those one-handed players, as it is a wave-based shooter, it will always have a guarantee of sexual content with whichever enemy you choose, rather than making you wait potentially hours to get the encounter you want, like in some RPGs or action platformers.
There are, of course, issues with the core mechanics of the game, but as a what is closer to a tech demo than a feature-complete game, I think it stands out far above the rest. The level and enemy variety is extremely well-done, and the clear attention to detail makes this $5 abandonware (hopefully only temporarily) an easy recommend.