Jutaijima

One male finds himself deserted on an island with numerous pretty women. A harem emerges in both cases, but while all of the girls in Boy Meets Harem are willing, that is not the case in Jutaijima, where the girls must fuck to stay alive.  

"oh no, because of a set of circumstances set by the plot we must receive the semen/impregnation of the protagonist, well there's no other way, so we may as well start right away!" That's basically the plot in both of these, there's differences of course, like the protagonist in Youkoso! Sukebe Elf No Mori E is the chosen one and in Jutaijima was the only male lucky survivor, or that in Youkoso! There's pure elves, but it makes up for it by both having the same studio and people behind the creation, so animation character designs, art, etc. Is very similar, so they're both great. 

If the main protagonist doesn't have sex with the female cast, they'll all die, well in Jutaijima is only the girls who'll die, but it's basically the same concept, by different ways and reasons, but same concept after all, coincidentally both are adapting a visual novel by their respective names and both adapt different routes, only that in Jutaijima the alternative route is the sequel. Lots of smut with sexy girls in both mainly students and a sensei, you know the deal. 

Besides the fact that in Boin the protagonist is a teacher and in Jutaijima a normal student and Jutaijima has much more darker stuff comparatively to the more mundane setting of Boin, both hentai have a similar variety of girls to chose from, which comes with various female students and a big chested sensei with glasses, this last one is really similar in both (Jutaijima has a blonde doctor too), there's also only one guy at play with these of ladies, so there's the harem aspect in both. 

Both are visual novel adaptations with the way of adapting being animating various routes from the original vn by themselves (the sequel Ura Jutaijima for Jutaijima), with the alternative routes more answers are responded in both and even a certain plot point from Ura Jutaijima is similar to twists that happened in the second season of Highrashi (Kai). Jutaijima is hentai, have that under your consideration.